Biology Form 4 Best Notes for all Topics

It’s a branch of Biology that deals with the study of inheritance. The science of genetics attempts to explain why organisms differ from one another and at the same time show similarities within the same species.
Members of the same family may differ in appearance of face, shape of the nose, ears, skin colour etc this is due to a variety of hereditary factors that each of them acquires from their parents.
Variation
It refers to observable differences among living organisms. There are two types of variations among individuals of a given species;
Discontinuous variation
In this type of variation there are definite/distinct groups of individuals with no intermediate forms e.g.
In human population an individual is either a male or a female
The ABO blood group system in man-an individual can only belong to one of the groups A,B, AB or O
Ability to roll the tongue into a U-shape-some people can roll their tongues while others cannot
The presence of long hair in the nose and in the ear pinna
Some people have a free ear-lobe while in others it’s attached.
Finger prints-In humans there are 4 main types of finger print patterns i.e. arch, loop, whorl and double whorl. Each individual inherits only one of the 4 main types of finger prints.
Ability to taste a chemical substance called phenylthiocarbamide (PTC).Some individuals are able to taste (tasters) while others are unable (non-tasters).
In plants, a pawpaw tree is either male or female
NB Discontinuous variation is basically determined by the genetic factors.
Practical Activities
- Activity 1-Investigating tongue-rolling in humans
Members of the class to try and roll their tongues
Count and record the number of rollers and non-rollers and tabulate your results

– Tongue-rolling	– Tally	– Frequency
– Tongue-roller	–	–
– Non- Tongue-roller	–	–

Activity 11-Investigating finger prints in humans

Place the tip of your right thumb on the ink-pad and press lightly on it
Press the inked thumb gently on the plain paper to make an impression of your thumb print. Members of the class to make a print on the same piece of
Observe the finger prints using a hand lens. Identify each print and name it using the figure below as reference.

– Finger print type	– Tally	– Frequency(No of individuals)
– Arch	–	–
– Loop	–	–
– Whorl	–	–
– Double-whorl	–	–

Continuous variation
This type of variation exhibits a wide range of differences for the same characteristic from one extreme end to the other e.g.
Height- Height within a group of people ranges from the shortest to the tallest with several intermediates.
Skin colour (pigmentation) – Some people are very dark- skinned while others have very light-skins with several intermediates
Body weights.
NB Continuous variations arise from the interactions between the genetic and environmental factors e.g. a plant possessing genetic factors for tallness may fail to grow due to unsuitable soil and climate.
Causes of variation
Gamete formation
Two processes contribute to variations;
Independent Assortment
During Metaphase1 the homologous chromosomes come together in pairs and subsequently segregate into daughter cells independently of each other. This produces a wide variety of gametes. The number of combinations is 2ⁿ where n is the haploid number of chromosomes. In man this is 2²³which is 8388608. That’s why even brothers are not exactly alike!
Crossing-Over
During prophase1 when the homologous chromosomes are in intimate contact with one another, the chromatids of the homologous chromosomes break and rejoin at certain points called chiasmata
During Crossing-Over important genetic exchanges take place at the chiasma resulting in more variations.
Fertilization
It permits parental genes to be brought together in different combinations. This way different quality of parents can be combined in the offspring.
Mutations
These are Spontaneous changes in the genetic make up of an organism
The Chromosome
Chromosomes contain the hereditary material or factors that are transmitted from the parents to the offspring. These factors are called genes.
There is a definite constant number of chromosomes in each cell for every species of animal or plant e.g.

– Species	– Common name	– No. of Chromosomes – Somatic Cells (2n)	– Gamete(n)
– Ovis auries	– Sheep	– 56 (28 pairs)	– 28
– Bos taurus	– Cow	– 60 (30 pairs)	– 30
– Drosophila melanogaster	– Fruit fly	– 8 (4 pairs)	– 4
– Zea mays	– Maize	– 20 (10 pairs)	– 10
– Pisum sativum	– Garden pea	– 14 (7 pairs)	– 7
– Mus musculus	– Mouse	– 40 (20 pairs)	– 20

GENES AND DNA
Genes occupy definite position on the Chromosome known as gene loci (gene locus).
The gene is chemical in nature and it is in the form of a nucleic acid molecule called Deoxyribonucleic acid (DNA).
DNA
It’s a complex molecule composed of 3 different components i.e.
a 5-Carbon sugar
a Phosphate Molecule
a Nitrogenous base- There are 4 types of bases i.e.
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)
A combination of the 3 above form a nucleotide. Nucleotides join together to form long chains called DNA strands. Two parallel DNA strands twist on one another forming a double helix.
Adenine always combines with Thymine (A-T) while Cytosine combines with Guanine (C-G) when forming the double helix structure of the DNA because they are equal. Therefore DNA is like a twisted ladder with Nitrogen bases forming the steps or rungs e.g.
The Role of DNA
Stores genetic information in a coded form.
Enables the transfer of genetic information unchanged to daughter cells through replication.
Translates the genetic information into the characteristics of an organism through protein synthesis.
DNA Replication
This is the process through which a DNA molecule forms an exact replica of itself.
Mechanism of Replication
DNA double helix consists of two long separate strands joined together by the base pairs. When the molecule is due to replicate the double helix unwinds and the two strands unzip themselves. This is made possible by the presence of the weak Hydrogen bonds that link the bases of the two strands.
After unzipping the information on the DNA strands (base sequence) is copied out onto a new DNA structure using the parent DNA as template e.g.
The replication makes it possible to pass on the DNA molecule together with its exact genetic information to the daughter cells during cell division in the organism.
ROLE OF DNA IN PROTEIN SYNTHESIS
The sequences of the bases along the DNA strands are important. It acts as the alphabet or code that spells out the sequence of the amino acids when they join up to form protein polypeptide chains.
The set of a base triplet is known as a codon and is said to code for a particular amino acid of a protein molecule e.g.
AAA-Codes for amino acid Phenylalanine
TTT- Codes for amino acid Lysine
The cell has got a special molecule that mediates between the DNA and the cytoplasm. This molecule is also a nucleic acid molecule and is known as Ribonucleic acid (RNA). Since its role is to carry genetic information from the DNA to the site of protein synthesis in the cytoplasm, it is referred to as Messenger RNA (M-RNA) and is formed from the DNA strands.
In the formation m-RNA an appropriate section of the DNA strand serves as a template. The double helix of the DNA unzips and free nucleotides align themselves opposite the template. The base sequence of the template is copied onto a new strand which then becomes an RNA strand.
In the RNA, Thymine is replaced by the base Uracil (U). The transfer of DNA base sequence onto the m-RNA strands is described as
After its formation, m-RNA leaves the nucleus with the full instructions from the DNA about the kind of Protein to be synthesized by the cell. This instruction is in the form of base triplets or codons which are used to assemble the amino acids on the protein polypeptide chains.

Information on the m-RNA is translated by ribosome and is used to assemble the amino acids into specific proteins molecules. Proteins molecules determine the inherited characteristics in organisms.
- The First Law of Heredity
- Mendel’s Experiments
An Austrian monk known as Gregory Mendel is considered to be the father of genetics. He carried out various breeding experiments and observed variations in different characteristics of the garden pea.
Mendel selected a group of dwarf plants & self pollinated them by dusting mature pollen grains onto stigmas of the same plant. He then collected the resulting seeds and planted them. He then noted that these seeds germinated and grew into dwarf plants only.
He repeated these experiments for several generations and observed the same results. This showed that dwarf garden pea plants could only produce only their own type i.e. they were pure breeds i.e.

In another experiment, Mendel selected tall plants and self pollinated them. He then planted the resulting seeds and observed that they grew into a mixture of tall and dwarf plants.
He took seeds from the tall offspring only and repeated the experiment, for many generations until he obtained tall plants only.
He then cross-pollinated pure bred tall garden pea plants with the pure bred dwarf variety. He planted the resulting seeds and observed that the offspring were all tall plants e.g.

He crossed two of these tall offspring and planted the resulting seeds. He observed that this 2^nd generation consisted of a mixture of tall and dwarf plants. He counted these plants and noted that the ratio of the tall to dwarf plants was approximately 3: 1 respectively.

Mendel concluded that the characteristics of an organism are determined by hereditary factors which occur in pairs. Only one of a pair of such factors can be represented in a single gamete. This is known as Mendel’s 1^st law, the law of segregation.
This law states “The characteristics of an organism are determined by genes which occur in pairs. Only one member of the gene pair can be carried in a single gamete”
Mendel’s successes can be attributed to the following;
He used favourable materials i.e. garden pea which is self-fertilised.
His study focused on particular traits.
He kept accurate data on all his experiments.
The pea plant he used had several observable contrasting characteristics (traits)
- Monohybrid inheritance
In his experiments, Mendel postulated that the inheritance of one characteristic like height in garden pea plant is controlled by a single pair of hereditary factors contributed by both parents. This type of inheritance is known as Monohybrid inheritance.
It involves the transmission of just one pair of contrasting characteristics (traits) e.g. tallness and dwarfness for height
A single pair of hereditary factors known as genes controls such a trait.
Genes occur in pairs on the chromosomes and such gene pairs are known as alleles.
The genetic constitution of an organism is known as genotype while the outward appearance of the organism is called phenotype.
The genotype of an organism is represented using paired letter symbols where the capital letter represents the dominant gene while the small letter represents the recessive gene.
- NB The conventional symbol for male is ♂ while that of a female is ♀
The inheritance of dwarfness in garden pea plant can be illustrated diagrammatically by a genetic cross e.g.
Cross between two pure-bred dwarf plants
Cross between two pure-bred tall plants
Cross between a pure-bred tall and pure-bred dwarf plants
NB In the genetic cross above, the male plant is tall and the female plant is dwarf. If the cross is reversed so that the female is tall and the male a dwarf, this is referred to as reciprocal cross. The F1 results will be the same for either cross.
However, when these F1 offspring are self-pollinated, they produce offspring that grow into a mixture of tall and dwarf plants. These offspring are known as 2^nd filial generation (F2 generation) e.g.
From the genetic crosses above, the following components of a genetic cross are obtained;
Parental phenotypes.
Parental genotypes- Crossing (X) should be shown between two genotypes.
Gametes –Must be circled.
The fusion process (fertilization).
The filial generation types.
An alternative method of making genetic crosses is a punnet square i.e.

When the allelic genes are identical as in TT and tt, the condition is known as homozygous. An individual with such condition is referred to as a homozygote.
When the allelic genes are not identical as in Tt the condition is known as heterozygous. An individual with such condition is referred to as a heterozygote.
An individual with genotype Tt will be phenotypically tall because the gene T is dominant over the gene The allele t is recessive.
A dominant gene is that which can express itself in both its homozygous (TT) and heterozygous (Tt) state, while a recessive gene is that which can only express itself in its homozygous (tt) state. Therefore the genotypic condition TT is referred to as homozygous dominant while tt is homozygous recessive.
The ratio 3 tall: 1 dwarf in the F2 generation is characteristic of monohybrid inheritance where one gene is completely dominant over the other. This is complete dominance. The monohybrid crosses are based on Mendel’s 1^st law, which states that, the characteristics of an organism are determined by internal factors (genes) which occur in pairs. Only one of a pair of such factors can be represented in a single gamete.
- Ratios and probability
The 3:1 ratio in monohybrid inheritance can be presented in the form of probability i.e.
¾ or 75% of the offspring are tall while ¼ or 25% are dwarf.
The inheritance of characteristics involves probability. The chance that a particular gamete will fuse with another is a random occurrence. In genetic crosses this is done by showing all possible fusions.
Similar monohybrid results as those of Mendel have been obtained by using a common insect, the fruit fly (Drosophila melanogaster). The fruit fly is a suitable organism for genetic study due to;
(i) The female lays many eggs resulting in a large number of offspring. This increases the sample size.
(ii) Have many observable characteristics that are distinct and contrasting.
(iii) It is easily bred in the laboratory with minimum requirements.
(iv) It has a short generation time i.e. 10-14 days. Therefore many generations can be studied in a short period.
(v) Offspring can be crossed with their parents at will (back crossing).
(vi) The flies are safe to handle because they do not transmit any known human diseases.
- Practical activities
- Homozygote X Heterozygote
Label 4 beakers 1-4
Put 50 blue beads into beaker 1
Put 25 blue and 25 red beads into beaker 2 and mix them up thoroughly.
Close your eyes and take a bead from each beaker. Put the beads together (offspring). Open your eyes.
If the offspring is homozygous (either blue or red) then put it into beaker 3 and if heterozygous then put it into beaker 4.
Repeat steps 4 and 5 until all beads have been transferred.
Count and record the number of;
(i) Homozygous offspring…………
(ii) Heterozygous offspring………
What do the beads represent? genes
What conclusion do you make from the results obtained in 7 above?
Assume that the characteristics shown by the blue bead is dominant and is represented by B.
Complete the cross below;
Parents
Genotype ………. ………
Gametes ……… ……….
Offspring (F1)
Genotypes …… ………..
Ratio ………. ………..
Heterozygote X Heterozygote

Put 50 blue and 50 red beads into each of the two beakers 1 and 2 and mix them thoroughly.
Close your eyes and then take a bead from each beaker. Put the two beads together.
If the offspring is homozygous dominant (both blue) put them into beaker 3 and if heterozygous put into beaker 4. if homozygous recessive (both red) put into beaker 5.
Repeat steps 2 and 3 until all the beads have been transferred, a pair at a time into beakers 3, 4 and 5.
Now count the number of;
- Homozygous dominant offspring…………
- Heterozygous offspring………
Homozygous recessive offspring………………
What conclusion do you make from the results obtained in 5 above?
Assume that the characteristics shown by the blue bead is dominant and is represented by B.
Complete the cross below;
Parents(heterozygous )
- Genotype …Bb …Bb……
- Gametes …Bb Bb…….
- Offspring (F1)
- Genotypes 1BB……2Bb 1bb
- Ratio …
Why is it important to to keep your eyes closed when picking the beads?
Incomplete dominance
In the experiments above, the tall plant with the genotype TT (homozygous) could not be distinguished from an individual with the genotype Tt (heterozygous). Thus although the genotypes were different, the phenotypes were identical, indicating that the gene determining tallness is completely dominant over that determining dwarfness.
However, in some plants such as Mirabilis jalapa (4 o’clock plant) a cross between red and white flowered plants results in F1 generation with all the offspring bearing pink flowers thus showing the blending of colour.
Selfing of F1 individuals gives a phenotypic ratio of 1 red; 2 pink; 1 white. This shows that there is no allele which completely dominates the other. This is an example of incomplete dominance.
NB in genetic crosses involving incomplete dominance, two different capital letters are used to represent the 2 genes. E.g.
Let R represent gene for red flower colour
Let W represent gene for white flower colour
- Selfing F1

An example of incomplete dominance in animals ;
In short-horned cattle, the gene for the red coat colour is co-dominant to white coat colour. A cross between them produces a mixture of red and white coat colour referred to as roan.
- Assignment
In a particular species of tropical beetle, the wings had either red or orange marks. A cross between red marked beetles with orange marked beetles produces offspring with yellow marks only. When the F1 offspring were selfed, they produced F2 generation in the ratio of 1 red; 2 yellow; 1 orange.
(a) Explain the absence of red and orange marks in the F1 offspring.
(b) Using a genetic cross show how F2 generation was obtained.
In a plant breeding experiment, a tall pea plant was crossed with a dwarf pea plant. All the F₁generation plants were tall. The F₁ generation plants were selfed that resulted in 325 dwarf plants out of 898 plants.
- (a) (i) How many plants in F₂generation were tall (1 Mark)

What was the dominant character in the cross above?
(b) Using letter T to denote the gene for height, work out the genotypes of F₂ generation (3 Marks)
Inheritance of ABO blood groups
In man ABO blood group system is an inheritable characteristic that is transmitted from the parents to the offspring.
The ABO blood groups are determined by 3 genes (multiple alleles) and they are responsible for the presence of antigen types on the red blood cells.
These are;
Gene A responsible for the presence of antigen A.
Gene B responsible for the presence of antigen B.
Gene O responsible for no (zero) antigen on the red blood cells.
Genes A and B have equal degree of dominance i.e. they are co-dominant and both express themselves when present together as in AB blood group. Gene O is recessive and will only express itself in the homozygous condition.
The genotypes for the 4 blood groups in the ABO system are thus formed by allelic pairs of genes e.g.

– Blood group (Phenotype)	– Genotype	– Antigens
– A	– AA, AO	– A
– B	– BB, BO	– B
– AB	– AB	– A&B
– O	– OO	– O(zero)

A marriage between a man of blood group A and a woman of blood group B. If both parents are heterozygous, they will produce;
A man who is homozygous for blood group A married to a woman who is homozygous for blood group B would produce;
A Mrs. Abuto claims that her child was wrongly identified at the hospital so that she took away the wrong baby. Both Abuto and his wife are blood group A. the child’s group is O. Is Mrs. Abuto justified in her claim? Make a genetic cross to explain your answer.

Inheritance of Rhesus factor

In human beings, possession of rhesus antigen makes one Rhesus positive (Rh+) and this is dominant over absence of Rhesus antigens which is Rhesus negative (Rh-). This is an example of complete dominance in man.
If a woman who is Rh- is married to a Rh+ man, when she becomes pregnant the child will be Rh+. The Rhesus antigens cross the placenta into the mother’s blood stream prompting the mother’s immune system to produce Rhesus antibodies.
When the antibodies get into the foetal circulation an antigen-antibody reaction takes place and the red blood cells of the foetus are destroyed (haemolysed). When the baby is born it is very pale.
In the 2^nd pregnancy the antibodies are more and cause a lot of damage to the foetal red blood cells resulting in death. This is called haemolytic disease of the new born (Erythroblastosis foetalis)
- Determining unknown genotypes
In the garden pea the gene that determines red colour is dominant over that determines white colour, hence a plant with red flowers may either be homozygous or heterozygous for this characteristic.
To establish the genotype it is crossed with a homozygous recessive plant. If the offspring bear red flowers then it indicates that the red flowered plant is homozygous (pure line) e.g.
If the offspring bear a mixture of red and white flowers in equal proportions giving a ratio of 1:1 this indicates that the red flowered plant was heterozygous e.g.
A cross between an individual of unknown genotype with an individual of recessive genotype is known as a test cross. A test cross where an offspring is crossed with one of its parent is known as back cross.
Unknown genotypes can also be determined by carrying out selfing experiments e.g. a phenotypically tall plant is either genotypically homozygous or heterozygous for this trait. If selfed and all its offspring are phenotypically tall, then the parental genotype is TT (homozygous dominant).
If after selfing both tall and dwarf offspring are produced in the 3:1ratio, then the parental genotype is Tt (heterozygous) e.g.
- Sex determination
Sex determination in higher animals is controlled by a specific pair of chromosomes e.g. in man, there are 23 pairs of homologous chromosomes in every body cell. The genes that determine whether a child becomes a male or female are located on the specific pair of sex chromosomes called X and Y chromosomes (named after their shape).
The remaining 22 pairs of chromosomes are called autosomes and are responsible for other inheritable characteristics.
A male human being carries XY (heterogametic) while a female carries XX (homogametic) combination.
After meiosis in a male, the spermatozoa can either contain the X or Y chromosome while the female ova will contain only the X chromosome.
The sex of a child is a chance occurrence because it depends on whether the ovum is fertilised by an X or Y chromosome therefore there is a 50% chance that fertilization can result in either XY (boy) or XX (girl) e.g.
In the fruit fly (Drosophila melanogaster) sex determination is exactly as in human beings i.e.
Males XY
Females XX
Practical activities; To study sex determination in human beings
Materials
2 coins of similar denomination
4 labels
Procedure
Obtain 2 coins
Label one side of the coin X and the other as Y
Label the other coin X on both sides
Spin the 2 coins simultaneously and note the letter on top when they stop. Enter the results in the table below. Repeat the procedure 50 times.

– Sperm	– Ovum	– Tally	– Totals
– X	– X	–	–
– Y	– X	–	–

Which is the approximate ratio of XX: XY?
In birds male is XX and female XY; and in some insects the female is XX and the male is XO; the Y chromosome is missing altogether.
Linkage
Genes located on only 23 pairs of chromosomes control all the thousands of characteristics in human beings. Those genes found on the same chromosome are called linked genes. All the linked genes constitute a linkage group.
Linked genes are inherited together and therefore do not segregate during meiosis.
Sex-linked genes
All genes located on the sex chromosome are said to be sex-linked and therefore are transmitted together with those that determine sex.
Most sex-linked are carried on the X-chromosome whereas the Y chromosome carries very few genes and is almost empty.
In human there are a few genes located on the Y chromosome which control the characteristics that are exclusively male e.g.
-Premature baldness
-Tuft of hair in the ear pinna and in the nose.
Characteristics controlled by genes located on the X-chromosome include;
-Colourblindness
-Haemophilia
These characteristics can arise in either males or females.
Sex-linked genes in man
Colourblindness
It’s the inability to distinguish red and green colours by some people. This trait is linked to the X-chromosome.
The genes that determine normal colour vision is dominant over that for colour blindness.
A marriage between a colourblind man and a woman homozygous for normal colour vision results in their daughters being carriers, but with normal colour vision. The daughters are described as carriers because they are heterozygous. All the sons of the two parents however are normal e.g.
Let N represent gene for normal colour vision
Let n represent gene for Colourblindness
Since the gene is linked to X chromosome, its alleles are represented as X^N and Xⁿ.
If a carrier daughter from the above parents married a normal man, some of their sons will suffer from Colourblindness while the daughters will be either carriers or homozygous for normal colour vision e.g.
The above example shows that the gene for Colourblindness is passed from the mother to the sons. This is because the only X chromosome the male offspring inherits is from the mother.
There are more male sufferers in a population compared to females. Females only suffer when in homozygous condition of the recessive gene.
Inheritance of Colourblindness through several generations can be illustrated using a pedigree. Pedigree is a record in a table form showing the distribution of one or more traits in different generations of related individuals e.g

Haemophilia

It’s also referred to as bleeder’s disease. The blood of a person suffering from the disease takes an abnormally long time to clot in the event of a cut, resulting in prolonged bleeding.

Haemophilia is caused by a recessive gene on the X chromosome e.g. if a normal man marries a carrier woman for Haemophilia, there is a probability of ¼ that if their child is a boy he will be haemophiliac and if a daughter she will be a carrier e.g.
Let H represent gene for normal clotting condition
Let h represent gene for Haemophilia
Both genes are located on the X chromosomes eg X^H or X^h
Assignment
If a haemophiliac man marries a carrier woman ;
(a) Use a genetic cross to work out the product of the above marria
(b) What is the probability that their daughter will be haemophiliac
(c) The above disorder is more common in males than females. Explain
X
Also X and Y chromosomes bring about the primary and secondary characteristics e.g. feminine characteristics are controlled by genes on the X chromosome e.g
- -Enlargement of the breasts
- -Widening of hips
- -Growth of pubic hair
- -Onset of menstrual cycle
In males secondary sexual characteristics include;
-Breaking of the voice
-Growth of pubic hair and beard
-Widening of shoulders (masculine characteristics).
Other traits influenced by Y-chromosome include;
- -“Porcupine man”- This was the historical man who had patches of hard spiny skin like a porcupine. This trait was passed to sons only.
- -“Hairy ears”- This is where a tuft of hairs sprout out from the pinna of the ear and in the nose. This trait is common in parts of India and is confined to males only.
- Premature baldness is limited to males only.
Effects of crossing-over on linked genes
Linked genes are carried on the same chromosome but some of the linked genes separate and are transmitted on different chromosomes. This happens during crossing-over when sections of chromatids of a bivalent intertwine and may break off.
Some of these sections get rejoined to different chromatids thus separating genes that were previously linked.
Fusion of the few gametes containing chromatids whose genes have changed places in this way will produce new combinations (Re-combinants).
However majority of the gametes that fuse contain chromatids whose gene linkage has not been interfered with by crossing-over e.g.
X KLB PG 28 Fig 1.26
Mutations
These are the sudden changes which occur on the chromosome or genes resulting in change in the individual’s genetic make-up. Such an individual is called a mutant e.g.
In man, a haemophiliac might suddenly be produced from quite normal parents.
Resistance to DDT by some insects e.g. mosquitoes.
Mutations are normally due tom recessive genes hence mutations occur naturally but are extremely rare. However they can be induced by certain factors of environment called mutagenic agents e.g.
Exposure to gamma rays, ultra-violet (UV) and a variety of chemicals such as colchicines and mustard gas.
Types of mutations
There are two types i.e.
- -Chromosomal mutations/chromosome aberrations
- -Gene mutations
Chromosomal /chromosome aberrations
Involve changes in the structure or number of chromosomes.
Types of Chromosomal mutations
Deletion
This occurs when some sections of homologous chromatids break off and fail to reconnect to any of the chromatids. In this case, these sections are completely lost and genetic material they contain is said to be deleted out.
The consequence includes gross interference in the structure and development of an individual.
Most deletions are lethal since the offspring may lose genes responsible for the synthesis of some vital protein molecule e.g.
X KLB PG 29 Fig 1.27(a)
Duplication
A section of chromatid replicates and adds an extra length to itself, this adds a set of genes e.g.
X KLB PG 29 Fig 1.27(b)
If the gene duplicated were responsible for certain traits, these traits may be over-emphasized in the organism.
Inversion
It occurs when a chromatid breaks at two places. When rejoining, the middle piece rotates and rejoins in an inverted position. This reverses the gene sequence along the chromatid e.g.
X KLB PG 29 Fig 1.27(c)
Inversion might bring closer together genes whose combined effects are advantageous or disadvantageous.
(iv) Translocation
This occurs when a section of one chromatid breaks off and becomes attached to another chromatid but of the non-homologous pair. Hence translocation involves movement of genes from one non-homologous chromosome to another.
X KLB PG 30 Fig 1.27(d)
- (v) Non-disjunction
This is the kind of Chromosome abnormality that is caused by addition or loss of one or more whole chromosomes. It occurs during anaphase of the 2^nd meiotic division when two homologous chromosomes fail to segregate and move on into the same gamete cell. This results in half of the gametes containing two of the same chromosome whilst the others have none e.g.
X KLB PG 31 Fig 1.28(c)
The fusion of the gametes with two of the same chromosome with a normal gamete of the opposite sex will result in an individual with three such chromosomes i.e. the normal homologous pair plus one extra chromosome (trisomic).
A number of human diseases are known to come as a result of non-disjunction e.g.
Down’s syndrome/mongolism
This is where there is an extra somatic chromosome number 21.
The term mongolism was applied to the disease because the affected individuals have slit-eyed appearance typical of the Mongolian race.
Other characteristics are;
Reduced resistance to infection.
Reduced physical and mental development.
Thick tongue.
Cardiac malfunctions
Short body with stubby fingers
Flat nasal bridge
Higher incidence is among children of older mothers above 40 years old and may be due to depletion of nutrients in the eggs.
Fathers aged 55 years old and above also have increased risk of producing offspring with Down’s syndrome.
Klinefelter’s syndrome
In this case, individuals have an extra sex chromosome hence they have a total of 47 chromosomes in their cells i.e. XXY (male) and XXX (female).
This results when a gamete with an extra sex chromosome fuses with a normal gamete from the opposite sex e.g.
X
Symptoms of Klinefelter’s syndrome
People with this disorder are externally male (XXY) but they have female features e.g.
Female-like breasts, a condition called gnaecomastia.
Testes are underdeveloped and produce very few or no sperms (infertility)
Reduced facial hair
Taller than average with signs of obesity.
XXX (females) are females who appear relatively normal in most characteristics.
Turner’s syndrome
It’s where an individual lacks one sex chromosome hence there are only 45 chromosomes (XO) or (YO) in the cells instead of the normal 46 chromosomes.
YO zygotes do not develop due to the absence of many vital genes.
Individuals with Turner’s syndrome (XO) are females that show underdeveloped female characteristics e.g.
-Are infertile due to lack of ovaries and small uterus.
-No breast development
-Short in stature
Other abnormalities arising from non-disjunction of the sex chromosomes lead to the genotype XYY. Individuals with this genotype are males and are known to have a predisposition for violence.
Polyploidy
This refers to the presence of more than two sets of chromosomes in a cell. It can come about due to the failure of a cell to divide after the 1^st stage of meiosis.
If it divides into two after the 2^nd stage of meiosis, diploid gamete results. Fusion of a diploid gamete with a normal haploid gamete of the opposite sex results in an individual whose cell has 3 sets of chromosomes i.e. triploid (3n).
If two diploid gametes fuse, the result is tetraploid (4n). This is what is called polyploidy. Polyploidy can also occur if the whole set of chromosomes doubles after fertilization.
Polyploidy is rare in animals but is common in plants e.g. in species of wheat and rice it has various advantages e.g.
Increased yields
Early maturity
Resistance to drought, pests and diseases
Polyploidy can artificially be induced by using chemical called colchicines which prevents spindle formation during mitosis thus leading to a cell with double the number of chromosomes (4n).
Gene mutation (point mutation)
A gene mutation arises as a result of a change in the chemical nature of the gene. The change may involve some alteration n the DNA molecule.
Types of gene mutation
Insertion
Its the addition of an extra base onto an existing DNA strand e.g. if the base Guanine (G) is inserted between the 1^st two Adenines (A) at the beginning of the DNA chain, the resulting M-RNA base triplet and the subsequent amino acid alignment will be altered e.g.
By inserting Guanine at the position indicated, no polypeptide chain is formed because none of the intended amino acids have been linked.
- (ii) Deletion
It’s the removal of a gene portion. If the base thymine (T) is deleted from its position at the indicated section of the DNA strand, the base sequence becomes altered at this point e.g.
This results in the alteration of the sequence of the amino acids on the polypeptide chain hence production of a wrong protein.
Substitution
This is the replacement of a portion of the gene with a new portion. If Adenine (A) is substituted with Guanine (G) on a DNA strand the base sequence is altered at this particular portion e.g.
Inversion
This is where the portion of the DNA strand rotates through 180^o. The inversion results in the alteration of the base sequence at this point e.g.
- The following are short messages (SMS) on a cell phone communication and can be used as analogies of gene mutation.

– Intended message	– Actual message	– Mutation
– Buy me a skirt	– Buy me a shirt	– Substitution
– Mary went shopping	– Mary went hopping	– Deletion
– This is my team	– This is my mate	– Inversion
– Auntie is staying	– Auntie is straying	– Insertion

(a) For each of these messages identify the type of gene mutation illustrated
(b) In the messages above show the changes that cause the distortion in the intended message
Disorders due to gene mutation
Albinism
This is a condition where the synthesis of the skin pigment melanin, fails and is characterized by;
- -A light skin
- -White hair
- -Pink eyes
Such an individual is described as an albino.
Melanin is a derivative of an amino acid phenylalanine and tyrosine and is synthesized through a series of reactions. Each of these reactions is controlled by a specific gene.
In albinism, one of these genes (designated by letter A) is substituted by a recessive gene (designated by letter a). gene a in the homozygous condition aa blocks in one or two places of the chain reactions involved in the synthesis of melanin, hence melanin is not formed resulting in albinism.
The genotype of an albino is homozygous recessive aa. A carrier for the trait is heterozygous with the genotype Aa and has normal skin pigmentation e.g.
X
The skin of an albino person is susceptible to sunburn and the eyes are sensitive to bright light.
Use of sunglasses and sunscreen lotions help them to lead a normal life.
Sickle-cell anaemia
Normal adult humans contain haemoglobin A in their red blood cells and have the genotype Hb ^AHb^A. In the sickle-cell anaemia, haemoglobin A is substituted by a different type called haemoglobin S. such patients have genotype Hb ^SHb^S.
Haemoglobin S is defective and has a marked difference from the normal one e.g.

– Normal haemoglobin (Hb ^A)	– Defective haemoglobin (Hb ^S)
– A position in each polypeptide chain is occupied by glutamic acid	– A similar position in each polypeptide chain is occupied by valine
– Does not easily crystallize in low O₂concentration	– Easily crystallizes in low O₂concentration
– The haemoglobin is efficient in O₂ loading and transportation	– The haemoglobin is not efficient in O₂ loading and transport
– Red blood cells carrying them have the normal biconcave shape	– Red blood cells carrying them have are crescent or sickle shaped.

Most of the individual’s red blood cells are therefore sickle shaped and the person frequently experiences oxygen shortage to the body tissues hence cannot carry out strenuous physical exercises.
The sickle-shaped cells are not able to squeeze through capillaries; thus they end up clogging blood vessels preventing normal blood flow. This blockage results in to severe pain in the joints, arms, legs and the stomach.
With close medical attention, sickle cells persons lead a relatively normal life. However, most deaths are due to infections from other diseases or damage to tissues.
A less serious condition is the sickle cell trai This is a heterozygous condition where less than half the number of red blood cells is sickle shaped. The rest of the cells are normal and are efficient in O₂ loading.
Individual with sickle cell trait experiences a mild case of anaemia but leads a normal life. Such individuals have an adaptive advantage in surviving malarial attacks as compared to those who have normal haemoglobin.
Inheritance of sickle cell trait
X
Haemophilia
It’s due to a recessive gene on the X-chromosome produced by gene substitution. The condition is caused by a haemophiliac gene that prevents the production of the necessary clotting factors especially clotting factor viii called antihaemophiliac globulin (AHG).
Remedies include the introduction of clotting factors e.g. factor viii and ix from blood donated by normal persons into haemophiliacs.
Colour-blindness
This is where an individual is not able to distinguish between red and green colours.
Colour-blindness is brought about by the presence of a gene in its recessive form causing total absence or shortage of the respective cones for colour perception.
The gene for colour vision is located on the X chromosome. Due to the occurrence of the gene on the X chromosome, there are more male sufferers than females.
Older parents transmit a slightly greater number of mutations to their offspring than younger parents. This increased chance maybe as a result of X-rays or other radiations used in medical diagnosis and treatment.
Achondroplasia
It’s a disease that is characterized by a shortened body, legs and hands. Its transmitted by a dominant gene thus both the homozygous dominant (AA) and heterozygous (Aa) individuals show the disease. Homozygous recessive individuals are perfectly normal.
Approximately 80% of achondrodystrophic dwarfs die within one year of birth but those that survive show normal mental development and can have children.
Effect of environment on heredity
The development of an organism depends on its genetic make-up or genotype and environment.
The interaction between the genotype of an organism and the environment will modify the phenotype of the organism.
The degree of modification or influence of gene expression by the environmental factors such as disease and food can be observed and measured e.g. the effects of protein and vitamins in the development of young children has been observed.
Mental development and performance depends on both the inherited patterns and the total effect of the environmental influences.
Practical application of genetics
Plant and animal breeding
Offspring resulting from a cross between two genetically dissimilar lines often posses’ beneficial characteristics not shown by either of the parents. This principle is known as hybrid vigour and it has been put to good use in plant and animal breeding e.g.
In cattle, Hereford, an English breed contributes high beef production and quick maturation. The boran from Kenya contributes disease resistance and the ability to feed and grow on dry pastures. A cross between Hereford bull and a boran cow yields a suitable hybrid with all these qualities.
Early maturity in both plants and animals.
Resistance to diseases e.g. Cassava resistant to cassava mosaic
Increased length of productive season e.g. chicken breeds with long egg laying durations and kales which can be harvested over a long period.
Adaptations to local conditions e.g. maize varieties for various ecological zones.
vi) Flowers such as roses, orchids etc are selectively bred for their Colour, shape and aroma.
Blood transfusion
For transfusion, we consider only the effect that the recipient’s antibodies in blood plasma will have on the donor’s antigens on the red blood cells. The plasma of the donor will be so diluted that it will not affect the red blood cells of the recipient. Blood typing is necessary before transfusion. This includes both the ABO and the Rhesus antigens.
- -Settling legal disputes
By matching the blood groups of a baby with those of alleged parents is sometimes possible to settle legal disputes about parentage e.g. an unmarried girl gives birth to a child and accuses a well known politician of being the biological father. The girl has blood group A, genotype AO and the baby is blood group O. The accused man has blood group AB. Could the accused man be the biological father?
A more advanced method of settling such disputes is to match the DNA of the baby to that of the alleged parents. Such a match will show regions where the base sequence of the baby matches perfectly with those of its parents. This is called DNA fingerprinting.
- -Crime detection
The DNA is unique to each individual. Through a series of genetic techniques, the pattern of DNA base sequence is prepared and produced in film. The pattern for each individual is very specific and therefore a “DNA fingerprint”.
At the scene of crime, a specimen from the suspect e.g. hair, blood, semen in the case of rape is obtained and DNA extracted from the developed hairs or blood cells.
By comparing this DNA pattern, it’s possible to isolate the culprit from several suspects.
Genetic counseling
This is the use of genetic information to advice couples who have hereditary diseases about the chances of their offspring inheriting the diseases. Hereditary diseases include;
-Albinism
-Sickle-cell anaemia
-Haemophilia
-Haemolytic disease of the newborn
Examples
Susan is a carrier of Haemophilia and is worried that her children will turn out to be haemophiliacs. What would you advice her if she marries;
a normal man
X (skip 5 lines)
She will have a normal daughter and son, carrier daughter and haemophiliac son.
a haemophiliac man
X (skip 5 lines)
She will have a normal son, carrier daughter, and haemophiliac son and daughter.
Advice; It would be better if she marries a normal man
A woman about to be married had a brother who died of sickle-cell anaemia. When samples of her blood were taken and placed in low oxygen concentration, her red blood cells became sickled. This means she is heterozygous. However, her prospective husband’s blood remains normal. What would you advise her about her children?
X
Some children will be normal while others will be affected in low oxygen concentration.
It’s advisable for people to go for medical tests for blood groups, hereditary diseases and defects to avoid complications in future.
(d) Genetic engineering
This deals with the identification of a desirable gene, altering, isolating and transferring it from one organism to another.
Genetic engineering has also made it possible to produce genetically modified organisms (transgenic) which have resulted in increased production in crops and domestic animals.
Genetic engineering has been applied in the following fields;
Farming
Genetically modified maize and Soya beans have been produced which have resistance against insect pests.
Attempts have been made to transfer the gene for nitrogen fixation from bacteria into cereal crops to increase yields without use of fertilizers.
Bovine somatotrophin is a hormone that increases milk production in cows.
Medicine
A strain of Escherichia coli has been made which makes insulin. The gene in man that codes for insulin is transferred to the bacterium thereby producing insulin that is purer and in large quantities.
Human somatotrophic hormone (human growth hormone) for treatment of dwarfism is now extracted from genetically modified strain of Escherichia coli.
Sheep have been genetically modified to produce milk which contains medicinal proteins used to relieve Haemophilia and emphysema patients.
Effective vaccines from the viruses can be made which contain only outer coats of viruses instead of weakened viruses. Viral coats are mass produced and used in the development of vaccines that do not pose a danger of causing the disease.
Some genetically modified plants e.g. bananas are being targeted for the production of orally administered vaccines against rabies and cholera.
Biological warfare-“genetic guns”
Bacillus anthracis causes anthrax in cattle, sheep and goats and through them can reach man. It forms spores which are not easily destroyed since they can withstand boiling, freezing and destructive digestive juices.
The bacterium affects internal and external parts of the body. If inhaled it can cause pneumonia with bleeding in lungs, ulcers of the stomach and intestines. Internal infections can cause death in 72 hours. Thrown into enemy territory it cause havoc.
Other micro-organisms used in warfare include;
Vibrio cholerae– causes cholera
Clostridium tetani- causes tetanus
Salmonella typhi– causes typhoid
Gene therapy
It’s the replacement of faulty genes with normal ones aimed at correcting genetic disorders.
In somatical gene therapy, genetically modified organisms e.g. viruses are used to carry the normal gene and introduce it into the affected tissue cells. The defective gene in the tissue cell is thereby corrected by the genes in the carrier. This is being used in the treatment of lung cystic fibrosis.
Cloning
This is a type of reproduction where a group of cells arise from a single individual cell without fertilization. The offspring are called clones and are genetically identical.
In the recent past, cloning was carried out in sheep. In this process, a nucleus from a fertilized ovum was removed and replaced with a diploid nucleus of a cell from the mammary glands of another sheep. The ovum was implanted into the sheep’s uterus.
The lamb, Dolly was a clone from the sheep which donated the mammary gland cell.
Cloning of plants is more common and successful. It has resulted in tissue culture techniques through which new varieties of crops such as pyrethrum and bananas have been produced.
Designs to produce human-like creatures (human cloning) to be reared as a source of human spare parts in surgeries and transplants.
Human genome
Genome is the total genetic content of any cell in an organism. It comprises of all genes on all the chromosomes. In human there are up to 100,000 genes. The human genome project aims at;
-Gene mapping
This is identifying specific positions occupied by specific genes on a chromosome e.g. Haemophilia is located on the X chromosome.
-Sequencing of gene
It involves analyzing DNA to reveal the order of bases in all chromosomes
Importance of human genome project
Identification of defective genes hence facilitating their correction.
Identification of genes that is susceptible to certain diseases so that individuals can take preventive measures

EVOLUTION

Evolution attempts to explain the origin, diversity and unity of life.
It’s a gradual change of living organisms from simple life forms to more complex forms over a long period of time.
The origin of life
Theories to explain origin of life
Special creation
This is a belief that is held by all the world’s major religions and also found in folklore.
This theory narrates that the whole universe and all that it contains was created by the action of a supreme being (God). To “create” means to bring into existence something out of nothing.
This belief holds the following views on the origin and nature of life;
- -That life was brought into existence by a Supreme Being
- -That life forms were created in a perfect form and have remained unchanged over time.
- -That this knowledge is based on faith and cannot be disputed
- -The belief cannot be contradicted by science since it is not scientifically testable.
Chemical evolution
This theory suggests that life probably began through a catalytic effect of light bringing together elements to form simple molecules such as water (H₂O), Ammonia (NH₃) and Methane (CH₄).
It can be theorized that, millions of years ago simple molecules such as hydrogen, oxygen, nitrogen etc in the universe combined in various proportions to form different simple compounds such as water and ammonia.
Further combinations of such compounds resulted in the formation of complex molecules e.g. DNA. Successive replication of the molecules led to the formation of simple forms e.g. viruses and bacteria.
The period during which the chemicals combined to form the 1^st living organisms is called the period of chemical evolution of life.
This theory holds the following views on the origin and nature of life;
- -Life came into existence through combinations of chemicals.
- -The initial life forms were simple and have changed over the years to form the present complex organisms. This is the principle of organic evolution.
- -Scientific evidence in the form of experiments, artifacts and fossils back this theory.
Its theorized that the universe came into existence some 15 billion years ago. Our solar system seems to have been formed about 6 billion years ago.
Our planet earth is supposed to have been formed 4 billion years ago from the debris of a burnt out star.
Since then, it has undergone cooling and structural changes on its outer crust providing an environment for the development of living organisms.

Evidence for organic evolution

Fossil records
Fossils are remains of ancestral forms that were accidentally preserved in some naturally occurring material e.g. sedimentary rocks.
The study of fossils is called palaentology.
The most common method of fossil formation is petrification i.e. changing into rock. This is usually possible for hard body parts such as bones, teeth etc
The fossils give direct evidence of the type of animals and plants that existed at a certain geological age.
The earliest fossil are of Monera, then Protoctista, Fungi, plants and finally animals.
The age of fossils can be determined through radioactive dating. For recent fossils, carbon-14 (C¹⁴) is used hence called radioactive carbon dating.
The age of very old fossils is obtained by determining the age of the rocks where the fossils are found using the potassium-argon method.
- Limitations of fossil records
There are several missing fossil records (missing links) due to;
- -Some parts or whole organisms decomposed.
- –Some were scavenged upon
- -Not all had conditions suitable for fossilization.
Distortion of parts during sedimentation which may give wrong impression of the structures.
Destruction of fossils by geological activities such as earthquakes, faulting etc
- Fossil record of humans
In his book “The Descent of man” Charles Darwin proposed that man descended from ape-like creatures. The earliest primate stock from which man is supposed to have evolved is Proconsul (20million years ago). The proconsul gave rise to Pongidae and
Pongidae gave rise to the present day apes while Hominidae gave rise to the human
The earliest hominids belonged to the genus Australopithecus. It seemed to have had an almost upright gait. No evidence of culture (fire-making, tool making) has been associated with it.
Its representatives discovered recently include;
- –Australopithecus anamensis– from L. -Turkana dated 3.9 million years old.
- –Australopithecus afarensis– Also called “Lucy” found in Hadar Ethiopia dated 3.6 million years old.
- –Australopithecus africanus– dated 3 million years old and Australopithecus robustus dated 2 million years old. Both were found in South Africa.
- –Australopithecus boisei– dated 1.3 million years old and was found by Mary Leakey at Olduvai Gorge in Tanzania.
- -Zinjanthropus boisei (nut –cracker man) dated 1.8 million years old found at Olduvai Gorge in Tanzania.
The next genus was Homo. The species in this genus are;
- Homo habilis (Handy man)
Discovered by Richard Leakey and kamoya kimeu at koobi fora (eastern part of L. Turkana) dated 2-1.5 million years old
There is evidence of tool making e.g. hand axes.
Cranial capacity (brain size) 700cm³.
- Homo erectus (erect man)
Discovered in Africa, Asia and Europe.
There is evidence of;
- Tool making
-Use of fire
-Some mode of communal life and work.

Had a prominent brow-ridge over the eyes.
Small stature
Small brain size (775-1200cm³)
Recently a skeleton of Homo ergaster nick named “Turkana boy” has been found dated 1.56 million years old.
Fossils of Homo neanderthalensis (Neanderthal man) has been found in Germany dated 300,000 years old.
- Homo sapiens (Rationalising man)
The present day species of humans is thought to have arisen some 130,000 years ago.
Another fossil form of Homo sapiens called “Cro-Magnon man” dated 40,000 years in Europe is almost identical to modern man.
- Characteristics
High intellectual capacity
Ability to vocalise sound or communicate through language.
Express moral judgements and develop ideas.
- Structural differences between apes and humans

– Ape	– Human
– Cranium enlarged; brain size 500cm3; low learning capacity	– Cranium greatly expanded; brain size 1350cm3; high intellectual capacity
– Large incisor and canine teeth useful in defense and killing prey; omnivorous	– Small incisor and canine teeth unsuitable for self-defense or for killing prey; omnivorous
– Locomotion quadrupedal (on 4 limbs)	– Locomotion bipedal (on 2 limbs)
– Forelimbs used for arboreal locomotion and walking; opposable thumb	– Forelimbs (hands) used for manipulation e.g. tool making; opposable thumb
– Hind foot has opposable toe for grasping branches	– Non-opposable toe in foot; used mainly for stability on the ground

Comparative Anatomy
This is comparing the form and structures of different organisms. Those organisms which show similarities suggest that they have a common or related ancestry.
Where one basic structural form is modified to give rise to various different forms the phenomenon is called divergent evolution.
Adaptive radiation– This is where the divergent forms originating from a single ancestral form may become adapted to different ecological niche in a given habitat.

Examples of adaptive radiation

Homologous structures
They are those parts that have a common embryonic origin but may be modified to perform different functions e.g. form and anatomy of the forelimb differ in various vertebrate groups while the basic pentadactyl (5 digit) structure is retained.
Examples of homologous structures
Forelimbs in vertebrates
The forelimb of a bat is modified to form a wing for flight.
The forelimb of a horse is elongated to enhance speed in running.
The forelimb in whale/turtle is modified to form a swimming flipper.
Forelimb of man modified for grasping/holding
Beak structure in birds
- Beaks in birds have a common embryonic origin and basic structure. Depending on the type of food and mode of feeding this basic structure is modified in length, shape and size e.g.
- -Short stout beaks are common in seed eaters e.g. weaver birds.
- -Long slender beaks for nectar feeders e.g. sunbirds.
- -Strong hooked beaks for carnivorous birds e.g. eagles
Foot structure in birds
- They are modified for various functions and habitats although they have a common embryonic origin e.g.
- -The webbed feet of ducks are used for wading.
- -Heavy clawed feet of eagles are used for grasping and tearing prey.

II Convergent evolution

This is where different structures are modified to perform similar functions. E.g.

Analogous structures
- They are those structures that have different embryonic origin but have evolved to perform similar functions due to the exploitation of the same kind of environment.
  - Examples of analogous structures
- Wings
  - The wings of birds and those of insects have different embryonic origin but both are adapted for flight.
- (ii) Eye structure
  - The eye structures of humans and that of octopus are similar but their embryonic origin is different.

III Vestigial
structures

They are those structures that have in the course of time ceased to be functional and therefore have become reduced in size or rudimentary.

Examples of vestigial structures
- Appendix in man has no digestive function while in herbivores e.g. rat, its well developed and functions as the cellulose-digesting part of the alimentary canal.
- The python and the whale have no externally visible hind limbs but their original presence is evidenced by the existence of a rudimentary pelvic girdle.
- The kiwi (flightless bird) of New Zealand has reduced wings beneath the body plumage.
- A vestigial tail is present in humans in the form of a much reduced coccyx.
- The nictitating membrane in the eye of the mammals is now reduced and functionless; but in birds and fish it can move over the eye and act as the 3^rd

Comparative embryology
The embryos of different vertebrate groups (fish, birds, amphibians, reptiles and mammals are morphologically similar during the early stages of development. This relationship is as a result of their common ancestry.
- The closer the resemblance between the early stage embryos, the closer is their evolutionary relationship. This theory is called recapitulation theory. It can also be atated as “ontogeny recapiturates phylogeny”

Geographical distribution of organisms
- The theory of “continental drift” supposes that at one time the present continents formed one large single land mass which later broke up and the parts drifted away from each other.
- Before drifting occurred there had been migration of animals from the centre of the land mass mainly fro Asia to Australia. After the drift, animals with a common ancestry became isolated and evolved into different species.
Examples
- Amazon forest in South America is inhabited with monkeys with long tails while in African forests there are the short-tailed monkeys.
- The panthers and the jaguars in the forests represent the cat family while in Africa we have the leopards and cheetahs and in Asia there are the tigers.
- The llamas of the Amazon forest and modern camels of Africa and Asia are thought to have had a common ancestry in North America. It’s believed that from their common points of origin, they migrated into different continents thus became isolated and evolved into different species.
- The process of initial migration and subsequent isolation by physical barriers such as oceans, dry deserts and mountains seems to have given rise to a wide variety of animal and plant types.

(v)Cell biology
The cells of all higher organisms show basic similarities in their structure and functions. Thus all these cells contain cell membranes and organelles such as ribosome, Golgi bodies, mitochondria etc.

They also have some biological chemicals in common e.g. ATP and DNA. This strongly indicates that all cell types have a common ancestral origin.
Also plant cells contain cellulose cell wall, cell sap, chloroplast etc. all these features are absent in animal cells.
Among animals blood pigments are of universal occurrence. These include haemoglobin (vertebrates and invertebrates), haemocyanin (moluscs and crustaceans) and chlorocruorin (annelids).
(vi)Comparative serology
Experiments with serum (serological tests) are used to show phylogenetic relationships e.g.
-If human serum is injected into a rabbit, the proteins in the serum act as antigens. The rabbit produces antibodies against the human proteins. When blood with antibodies is drawn from the rabbit and mixed with serum from different animals, an immunological reaction occurs forming a precipitate. The amount of precipitate formed varies from one animal to the other. The greater the amount the closer the phylogenetical relationship between the animal and the human being.

Mechanism of evolution

Lamarck’s theory (Theory of use and disuse)
This theory was proposed by Jean Baptiste de Lamarck (1815). He proposed that when the environment demanded the need for a particular structure in an organism, the organism develop it in response to the demand. This led to the natural use and disuse of structures producing changes in the individual during its life time.
- He further proposed that these changes or structures that were acquired during the life time of an individual were then transmitted to their offspring and subsequent generations resulting in the emergence of new forms or new species.
Examples

He cited the example of development of the long neck of the giraffe which developed as a result of stretching upwards by some short-necked ancestral forms towards the higher levels of vegetation in competition with other browsers for food. The longer neck trait developed was passed on to successive generations giving them advantage over the short-necked forms.
- The appearance of flightless birds e.g. ostriches, kiwi etc. their reduced and functionless wings resulted from their disuse in an environment not requiring flight.
- NB This theory was rejected because phenotypically acquired characteristics which do not affect the genotype of an individual cannot be inherited.
Darwin’s theory of Natural selection
- This theory was proposed by Charles Darwin (1859).
Main features of the theory
- He observed that variations arose by chance and from within the individual. Through sexual reproduction the characteristics are transmitted to successive offspring. Some of the variations confer an advantage to the individual while others are disadvantageous.
- He proposed that in nature there exist a phenomenon known as “Natural selection” which “selects” those individuals best suited to an environment.
- He observed that the number of offspring by far outnumber the parental generation but due to eliminating factors such as predation, diseases, competition for food, breeding conditions etc only a few survive to adulthood and are able to reproduce. He explained that in nature there exists a “struggle for existence” and those best suited for an environment survive. He called this “The survival of the fittest”.
- He proposed that “Natural selection” is a chance occurrence. Neither the environment nor the individual controls the direction of change.
- Those individuals that are selected for reproduction thus transmit the variations to their offspring. The gradual accumulation of small variations from generation to generation over a long period of time leads to the emergence of new forms of species.

Natural selection in action
- Strains or varieties could offer evidence of evolution in progress e.g. i
  - Peppered moth (Biston betularia)
- This moth occurs in Britain. They occur in two forms i.e. speckled white form and black melanic form.
- Before the industrial revolution, the speckled white form was well camouflaged against tree trunks with similar pattern and colouration. But around 1848 during industrial revolution the environment was darkened by soot. Due to this change the black melanic form blended well against the bark of trees which had been darkened by soot and smoke.
- These darker forms were better protected in the industrial area against predation from birds than the lighter forms.
- In the smoke and soot polluted areas there is greater frequency of black melanic form while in the soot free areas the speckled white form predominates. Thus the agent of natural selection was selective predation on the unprotected forms.

g. ii Sickle cell trait
It’s known that there is a high frequency of this mutant gene in places where malaria incidence is high. This is because those who are heterozygous HbAHbS have immunity to malaria a situation called heterozygous advantage.
g. iii Resistance to drugs, pesticides and antibiotics
Some pathogens and pests usually survive and reproduce even when they are exposed to the antibiotics and pesticides repeatedly. These instances show that within the population some individuals posses the gene for resistance or acquire it through mutation hence survives the chemical. Those that survive transmit this characteristic to their offspring thus establishing a new population of resistant forms.
-Mosquitoes posses a gene that makes them synthesise an enzyme against DDT.
-The control of plasmodium has been made difficult due to the ability of the plasmodium to rapidly change its coat surface antigens. This makes the plasmodia resist the drug meant to kill them.

RECEPTION, RESPONSE, CORDINATION

Sensitivity / irritability – it’s the ability to detect and respond to changes in the environment.
Stimuli (singular stimulus – it’s a variation in condition which can produce a change in activity in part or the whole organism.
Response – it’s a change in activity by the organism
Receptors – the part of the body which receives stimuli
Effectors – those parts of the body that bring about response.
In order for sensitivity and response to be effected there must be receptors to receive the stimuli, coordinators to integrate information received transmission system to conduct the stimuli &effectors to respond to the stimuli e.g.
Response to a variety of stimuli
Responses can be grouped according to the type of stimuli. This may involve the movement of the whole organism or a part of it in response to the stimuli. If the movement is towards the stimuli, then it is called a positive (-ve) response but if it’s away from it its called negative responseE.g.
Taxis – this is a locomotory response of a motile cell e.g. gamete or a whole organism in response to an external stimuli. The stimuli is unidirectional i.e. a stimuli from one direction. The responses are grouped according to the stimuli which cause them.
Phototaxis – this is a response to variation in light intensity and direction e.g. when Euglena, spirogyra and fruit flies move towards light; wood lice, maggots and termites usually move away from the light.
Aerotaxis – this is response to variation in oxygen concentration e.g. amoeba moving from an area of low oxygen concentration to high oxygen concentration.
Osmotaxis – response to variation in osmotic pressure as shown by marine crabs burrowing in the sand to avoid dilution of the body fluids.
Rheotaxis – response to variation in direction of water or air currents.
Fishes and planarians move against currents in water while butterflies and moths fly into wind currents in order to detect the scent of flowers.
Chemotaxis – response to variation in chemical substance e.g. movement of male gametes towards the female gametes. Sperms (antherozoids) of mosses and ferns are attracted to move towards chemical produced by the ovum in the archegonia. Mosquitoes will fly away from insecticide repellants.
Thermotaxis – it is the locomotory response o temperature (15⁰c) to moderate warmth (25⁰c)
Survival value of tactic responses
Enable organisms escape from harmful stimuli e.g. excessive heat, predators.
Organisms are able to seek favorable habitats and acquire resources e.g. nutrients, mates e.t.c.
Chemo taxis enables fertilization to take place
Reception, response coordination in plants
The sensitivity of plants is brought about by responses on part of the plant. This response is in form of growth movement or tropisms.
Tropisms
This is a growth movement of parts of plants in response to unidirectional external stimulus.
The growth movements are often slow because growth rate is usually controlled by plant hormones(auxins)
Types of Tropisms
Phototropism
This is a growth curvature in response to the direction of intensity of light. Shoots are positively phototropic while roots show negative phototropism.
Chemotropism
This is a growth curvature in response to a gradient of chemical concentration e.g. pollen tubes grow towards chemicals secreted by the embryo sac.
Geotropism
This refers to the growth curvature in response to gravity. Roots are positively geotropic while shoots are negatively geotropic.
Hydrotropism
It refers to the curvature in response to water or moisture. Plant roots are positively hydrotropic.
Haptotropism/ Thigmotropism
This is growth curvature in response to contact with a solid object. It is shown by tendrils or climbing stems which twine around objects e.g. branches or tree stems.
Root tips show negative thigmotropism when they grow avoiding solid obstacles such as rocks.
Survival value of tropic responses
Phototropism exposes the leaves in position to maximize light absorption thereby enhancing photosynthesis.
Hydrotropism enables the roots of the plant to seek water.
Haptotropism enables the plant to obtain mechanical support especially in those plants lacking woody stems.
Geotropism enables plants roots to grow deeper into the soil thus offering firm anchorage to the plant.
Chemotropism enables the pollen tubes to grow towards the embryo sac thereby facilitating fertilization.
Comparison of tropic and tactic responses.
Tropism and taxes are both adaptive responses that enable the organism to survive better in their environments.
Both responses are due to similar external stimuli such as light, water temperature.
Both responses are due to unidirectional stimuli.

– Tropisms	– Taxes.
– i. Results in growth curvature responses which are more permanent	– Results in Locomotory responses which are temporary.
– ii. Responses are slow	– Reponses are fast
– iii. Are brought about by influence of growth hormones	– Absence of hormonal influence.

(b) Nastic responses
These are non-directional movements of parts of plants in response to diffuse stimuli. Such responses include folding of leaves in hot weather, opening and closing of flowers in response to intensity of light, closing of leaves of Mimosa pudica when touched.
These movements are brought about by turgor pressure changes at the leaf and petal bases of certain plants. At these bases there are pressure sensitive swellings called pulvini which through loss or gain of turgidity bring about these movements.
- Types of Nastisms
Nyctinasty (Sleep movement)
These are movements in response to differences in light intensity and temperature changes of the day and night e.g. sunflower.
If the response is specifically for light then it is called photonasty where it opens in the presence of light and close in its absence.
If the response is specifically for temperature changes then it is called thermonasty.
Haptonasty
This is the response to touch e.g.:-
(i)Mimosa pudica:- The leaves of this plant will close rapidly if they or their stem are touched. A sudden change of temperature will initiate a response.
(ii)Venus fly-trap (Dionaea)- This is an insectivorous plant that grows in soil deficient in Nitrogen so it gets its Nitrogen by trapping and digesting insects. When the sensitive (trigger) hairs on the leaves are touched by a landing insect, the mid-rib cells lose water rapidly causing the trap to spring hence closing the leaf with the spines interlocking.
Chemonasty
This is the response to the presence of specific chemical substances of nitrogenous compounds such as urea and ammonium compounds found in insectivorous plants e.g. sundew (Drosera)
When an insect is trapped by the tentacles of Drosera, the insect provides the chemical stimulus for the release of digestive enzymes by the plant.
This is a response to changes in humidity. This type of response is seen in some flowers e.g. Dandelion genus which close when the air is moist.
Survival value of nastism
Protection of the inner delicate parts of the flowers.
Reduction of transpiration
Regulation of temperature
A way of obtaining some limited mineral nutrients
- Coordination in plants
- Role of auxins in Tropisms
Auxins are a group of plant growth hormones and one of the commonest auxins is indole-acetic acid (IAA). Auxins are produced at the apical meristems of the shoots and roots.
IAA stimulates growth in both shoots and roots at the region of elongation. Roots are more sensitive to auxins than shoot i.e. requires smaller concentration to stimulate growth compared to shoots.
- Auxins and phototropism
Under uniform light distribution auxins produced from the shoot apex are translocated evenly down the shoot therefore there is equal growth rate in the height of the shoot. E.g.
When a shoot is exposed to unidirectional light, the shoot tip bends towards the light source. This is because light causes lateral migration of auxins from the lit side of the shoot to the darker side. This leads to higher concentration of the auxins on the darker side. This higher concentration of the auxins stimulates rapid cell elongation and hence faster growth rate than the lit side. Eventually the shoot curves towards the source of light hence the positive phototropic response to light in shoots e.g.
- Auxins and geotropism
If the seedling is placed in a horizontal position in the dark, it has greater accumulation of auxins on the lower side.
Gravity causes a greater concentration of auxins to migrate and accumulate on the lower side of growing stems and roots. In the stems such high auxins concentration promotes faster growth but in roots it inhibits growth. Therefore greater auxin concenntation on the lower side in the shoot promotes faster growth on the lower side than on the upper side causing the shoot to bend upwards; while in the root lower auxin concentration on the upper side promotes faster growth on the upper side than on the lower hence the root bends downwards. Eg
- Auxins and thigmotropism
In plants when climbing stems or tendrils come into contact with a suitable hard object, the contact causes them to curve and coil round the hard object. The part of he stem in contact with the hard object has a lower auxin concentratation than the outer part. Contact causes lateral migration to the outer side of the stem.
Since the higher auxin concentration promotes faster growth in shoots, the greater auxin concentration in the outer part causes faster growth than the part in contact with the object hence the shoot continues to coil round the object.
- Co-ordination in animals
Irritability in animals is caused by the nervous system.
In arthropods the nervous system consists of peripheral nerves and a ventral nerve cord.
In higher animals such as vertebrates, irritability is brought about by a more elaborate nervous (neuro-sensory) system and the endocrine (hormonal) system. The nervous (neuro-sensory) system provides the quickest means of communication in animals.
- Nervous systems in mammals.
It consists of:
Central Nervous System (CNS)
It is made up of the brain and the spinal cord It receives and integrates impulses from the receptors and then relays them to the effector organs. It is the centre of coordination
Peripheral Nervous System
It is made up of sensory nerves and transmits nerve impulses from the receptors in the sensory organs to the CNS and motor nerves that transmit impulses from the CNS to the effector organs.
Structure and function of Nerve Cell
The nerve cell (neurone) is the basic functional unit of a nervous system. It’s a cell which is modified to transmit impulses
A nerve impulse is an electric signal that is transmited along a nerve fibre.
The neorone consists of :
The cell body (Centron)
Extensions called dendrites
In some neurons one of the dendrites is elongated to form an axon.
Each axon is filled with a specified cytoplasm called axoplasm, which is usually continous with the cytoplasm of the cell body.

The axon is enclosed by a fatty sheath called myelin or medullated sheath. The myelin sheath is surrounded by the neurilemma which is the membrane of the schwann cell.
Myelin sheath is broken at one millimeter intervals by constrictions called nodes of ranvier.
Nodes of ranvier help to propagate the nerve impulse and speed up the transmission of an impulse.
Myelin sheath helps to insulate the axon.
Neurones are described as unipolar, bipolar or multipolar according to how many dendrites project from the cell body. Eg.

The dendrites of the cell body make contact with neighbouring neurones in the CNS while the terminal dendrites at the end of axon make contact with effector organs.
Types of Neurones
There are three types of neurones grouped according to the direction of impulse conduction i.e.
Sensory Neurones (Afferent)
This nerve cell links the sense organs such as the ear, eye skin, nose and tongue with the CNS.
Its cell body is situatsd off the axon and outside the C NS.
Its receptor dendrites are located in the sense organ while the terminal dendrites are located in the CNS
Its function is to transmit nerve impulse from sense organs to the CNS
- Motor neurones
This nerve cell links the CNS with the effectors such as muscle fibres and glands.
Its cell body is located at one end of the axon with the CNS
its motor end plates terminate in a muscle or gland.
It transmits nerve impulses from the CNS to the effectors.
- Relay neurone
This neurone links a sensory nerve with a motor neurone through small gaps or neural junctions called synapses. It’s therefore a bipolar or multi-polar nerve.
The entire neurone is located within the CNS.
They are non-myelinated.
The main function of a relay neurone is to relay nerve impulses between sensory and motor neurone.

Central Nervous System (CNS)
Its composed of:
The brain
It’s encased in a bony structure called the skull or cranium.
It is enveloped by a system of protective membranes called meninges.
The meninges consist of:
Dura matter.
It’s the tough outer membrane covering the brain and the spinal cord of vertebrates.
It’s composed of connective tissues and rich network of capillaries.
Pia matter
It’s the inner most membrane covering the brain and the spinal cord.
It possesses many blood capillaries and lymph vessels.
Arachnoid layer
It is a narrow space between the dura and pia matter.
It is filled with cerebrospinal fluid, from which oxygen and nutrients diffuse into the brain cells
Within the brain there is a system of cavities called ventricles which are filled with cerebrospinal fluid. This fluid is continuous with the spinal fluid of the central cord of the spinal cord. This fluid is similar to lymph.
It provides nourishment to brain tissues
Serves as a shock absorber from mechanical damage.

Functions of major parts of the brain

The brain is divided into three major regions i.e.
Fore brain
Largest part of the brain about .⅔ of the brain
It consists of:
Cerebrum
Thalamus
Hypothalamus
Pituitary gland
Cerebrum
It is a highly developed part consisting of left and right cerebral hemispheres. It is important for;
Integration of sensory impulses such as vision, hearing and taste- responsible for emotions i.e. joy and sorrow
It controls voluntary body movements e.g. Limbs, lips and neck.
It also controls learning, memory and human individuality, imagination and intelligence, thoughts and reasoning
NB Plasmodium may enter the brain causing cerebral malaria leading to mental disorder.
Thalamus – it contains receptors for pain and pleasure
The center for integration of sensory information
Hypothalamus – it is located below the thalamus.
It has receptors for homeostatic functions such as thermoregulation and osmoregulation.
It controls appetite, thirst and sleep.
Pituitary gland – it is attached to the hypothalamus and projects downwards from it.
It is the master endocrine gland controlling the rest of the endocrine glands.
Mid Brain – (Corpora quadrigemia)
It is the connecting stalk between the fore brain and hind brain.
It relays impulses between nerves from the spinal cord and the fore brain
Hind brain
The major parts of the hind brain are:
Cerebellum
The main function is maintenance of body posture and balance. This is achieved by controlling and coordinating muscular movement.
Dexterity in fine movements.
Medulla oblongata.
Controls involuntary activities such as breathing, swallowing, salivation and vomiting.
Controls dilation or constriction of blood vessels thereby influencing blood pressure
Spinal cord.
It is the posterior extension from the brain to the tail.
It is enclosed in the meninges and protected by the vertebral column.
It’s made up of grey matter and white matter.
The grey matter is H-shaped and surrounds a central canal which is filled with cerebrospinal fluid. This grey matter relays information between the sensory & motor neurones.
It consists of cell bodies & dendrites of relay and motor neurones which give it its darker appearance hence the term “grey” matter.
White matter– it surrounds the grey matter and consists of the sensory & motor neurones.
The myelin sheath of these neurones gives this part its shiny white appearance.
Arising from the spinal cord are dorsal and ventral roots of spinal nerve.
Transverse section of the spinal cord

PERIPHERAL NERVOUS SYSTEM

It’s made of
Cranial nerves
They arise from the brain and form part of peripheral nervous system associated with receptors & effectors in the head.
In human beings there are 12 pairs of cranial nerves confined to the head and neck. Examples of cranial nerves are optic, auditory, facial and olfactory nerves.
There are also sensory & motor nerves which innervate the jaws and face.
The 10^th cranial nerves, vagus nerve, innervate the heart, lungs, diaphragm and the gut.
Spinal nerves
They innervate the skeletal muscles of the limbs and trunk.
In association with cranial nerves they control all the organs of the body below the head e.g. urinary bladder, the gut, liver, kidney and lungs.
Reflex action
It’s a rapid automatic response to a certain stimulus. There are two types of reflex action
- Simple reflex action
In a simple reflex action there is a specific single automatic response to a particular stimulus. It is the simplest form of reflex and does not depend on learning
Examples
Withdrawal of a finger from the hot or sharp object.
Blinking of the eye when an object passes close to it.
Coughing
Sneezing
Knee jerk reflex when the knee is tapped
Salivation
Secretion of tears when an onion is cut.
The structural basis of a reflex action is called the reflex arc, which is the pathway followed by the nerve impulse. The simplest reflex arc is made up of the 3 neurones; the receptors (sensory), relay and motor. They link the receptors with the effectors through the spinal cord.
When somebody accidentally touches a hot object the pain receptor in the skin are stimulated. This generates an impulse which is conducted by a sensory neurone to the spinal cord. Here the impulse is passed to the motor neurones via the relay neurones.
The impulse travels along the motor neurones to the biceps which contract resulting in the withdrawal of the hand from the painful stimulus.
The sensory neurone is also connected to an ascending neurone (longitudinal inter-neurones) which transmits impulse to the brain. This makes one to become aware of the pain a fraction of a second after withdrawal of the hand.
- CONDITIONED REFLEX
This is as automatic response which can be evoked from an animal by unrelated stimulus substituted for one which normally elicits the response.
The 1^st experiments on conditioned reflex were carried out by Russian scientist Ivan Pavlov in 1902 using dogs.
Ordinarily the sight or smell of food initiates salivation in dogs. This is a normal reflex action called the salivation reflex.
In this experiments Pavlov rang a bell whenever he was feeding his dogs. He continued doing this for several weeks and the dogs learnt to associate bell ringing with food.
Later on he rang the bell in the absence of food. He found out that this stimulated salivation in dogs, thus the original stimulus (sight or smell of food) was replaced by a different and unrelated stimulus (Ringing the bell) through learning.
A conditioned reflex usually weakens with time therefore it must be reinforced by repeated stimulus. This forms the basis of learned behaviour.
Examples of conditioned reflex
Walking
Playing
Cycling
Writing
Swimming
Driving
Everyday practical applications of conditioned reflex action include. Training of dogs, learning processes.
Differences between conditioned and simple reflex

– Simple reflex action	– conditioned reflex action
– i) single stimulus to bring about response	– Repeated stimulus to bring about response
– ii) Simplest form of behavior and is independent of experience.	– Involves modification of behaviors depending on experience.
– iii) sensory and motor components are the same at all times	– Primary and sensory components are replaced by a secondary component but the motor component remains unchanged.

Transmissions of nerve impulse

A nerve impulse is an electrical charge or wave of electrical disturbance arising from changes in ionic concentrations across the surface membrane of a nerve fibre (axon or dendrite).
The ions involved in impulse transmission are sodium ions (Na+) and potassium ions (K+)
- Resting Potential
A non- conducting nerve fibre is described to be in a resting potential. In this state there is more Na+ outside the axon membrane than inside in relation to the concentration of K+ which is higher within the axoplasm.
There are also relatively more anions (negatively charged ions) within the axoplasm. The net effect of this unequal distribution of ions is that there is positive charge outside the axoplasm and negative charge inside the axoplasm so that the membrane is said to be polarised.
During resting potential, Na+ are actively pumped out by a mechanism called sodium pump e.g.
- Action potential
It’s a localised change in electrical potential between the inside and the outside of the nerve fibre when stimulated. The inside becomes positively charged while the outside becomes negatively charged. This is called depolarisation e.g.
The membrane becomes more permeable. The sodium pump ceases causing an influx by diffusion of Na+ into axoplasm. This raises the concentration of Na+ ions within the axoplasm relative to the outside, causing the k+ ions to diffuse out.
This localised charge stimulates the depolarisation of the membrane adjacent to it, thus propagating the depolarisation process. This is immediately followed by the recovery to the polarised state. The movement of this action potential along a nerve fibre constitutes an impulse.
- Synapse/ Neuro-junction
A synapse is a point at which two nerve cells come into contact. At this point, a dendrite from one of the nerve cell forms an enlargement called a synaptic knob.
The function of the synapse is to allow the transmission of nerve impulses from neurone to neurone.
The transmission of impulses across a synapse is a chemical process that is mediated by chemical substances called neuro-transmitter substances.
The synaptic knob contains numerous sac-like structures called synaptic vesicles and mitochondria. The vesicles contain neuro-transmitter substances.
Mitochondria supply the energy necessary for continuous synthesis of neuro-transmitter substances.
The terminal part of the synaptic knob is called pre-synaptic membrane.
The membrane of the adjoining nerve cell is called the post-synaptic membrane.
Between the two membranes is a gap called the synaptic cleft.
- Transmission of an impulse
When an impulse reaches the synaptic knob, it stimulates the vesicles to move towards the pre-synaptic membrane releasing neuro-transmitter substances (acetylcholine). Acetylcholine makes the membrane permeable.
Acetylcholine diffuses across the synaptic cleft to the post-synaptic membrane which then becomes depolarized.
Na+ ions from the cleft then flow through the post-synaptic knob causing an action potential here.
The action potential is then transmitted as a nerve impulse along the neurone.
Immediately afterwards acetylcholine liberated in the synaptic cleft is destroyed by an enzyme called cholinesterase into inactive end products (choline and acetic acid (ethanoic acid).
These are then reabsorbed by the axon terminals and reconstituted into acetylcholine using energy in the form of ATP provided by mitochondria.
The rapid breakdown of acetylcholine is necessary to repolarise the pre-synaptic membrane for the next nerve impulse propagation so that there is no merging of successive nerve impulses from neurone to neurone.
- Accommodation of synapses
If a synapse is stimulated continuously for a long time, a point comes when no impulses are transmitted in the post-synaptic neurone.
The synapse is then said to accommodate or adapt to the stimulus e.g.
When one wears a rough shirt, an unpleasant sensation is initially felt. After sometime the sensation is not felt any more.
Accommodation is thought to result from exhaustion of the neuro-transmitter substance which cannot be synthesized as fast as it is required.
- Synaptic inhibitors
These are the substances that interfere with transmission of nerve impulses across the synapse e.g.
Atropine and curare block the post-synaptic membrane preventing it from being stimulated by neuro-transmitter substances
Organophosphates e.g. malathion inhibit enzyme cholinesterase. This prevents the destruction of acetylcholine leading to overstimulation of the post-synaptic membrane
Endocrine system
The system comprises of endocrine glands that are ductless and secrete hormones.
Hormones are organic compounds which are either protein or steroids in nature. They are produced in minute quantities in cells in one part of the body and transported by blood stream to the other parts of the same organism where they produce the response.
Those parts of an organism that respond are called target organs.
Hormonal feedback mechanism
g. if thyroid gland is producing too much thyroxine hormone, the signal level will be sent to the pituitary gland to secrete less thyroid stimulating hormone (TSH) also called thyrotrophin. The amount of thyroxine therefore falls.
This coordination is called negative feedback mechanism.
The hormones produced by the human body are:
Thyroxine
It is produced by the thyroid gland found at the neck region. It is a compound of iodine.
Functions
Controls basal metabolic activities by increasing glucose oxidation
Enhances the effect of growth hormone (Somatotrophin). This ensures normal growth and mental development.
It also works in conjunction with adrenaline to enhance involuntary activities such as increased circulatory rates.
Effects of under – secretion (Hypothyroidism)
It leads to insufficient iodine in the diet or defective enzymatic reaction concerned with its formation.
It leads to:
Cretinism in children
-The children have deformed legs
-Dry leathery skin
-Large tongue
-General body sluggishness
-Poor mental development resulting in low intelligence
Myxoedema in adults
-Swelling of the thyroid gland called goitre. This is due to overworking of the thyroid gland in an attempt to synthesize enough thyroxine
-Due to the low thyroxine concentration, individuals have low metabolic rate as shown by reduced heart beat, breathing rate and body temperature.
-They are mentally and physically sluggish
-The low physical activity results into weight gain (obesity), retention of excess fluid (oedema) hence swollen feet and puffy face.
Hypothyroidism can be controlled by use of balanced diet supplemented by iodized table salt and administration of iodine tablets.
Effects of overproduction of thyroxine(hyperthyroidism)
Hyperthyroidism is due to the presence of plasma proteins that stimulate the thyroid activity i.e. defective enzymatic reactions.
Leads to increased metabolic rate resulting in increased heart beat, breathing rate and high temperatures.
Individual show nervousness, restlessness and are easily irritable
Extreme hyperthyroidism can lead to heart failure, a condition known as thyrotoxicosis.
Can be controlled by treatment with radioactive iodine.
Surgical removal of parts of the thyroid gland can also be done.
Adrenaline
Produced by medulla part of the adrenal glands located above the kidney.
It prepares the body for emergency – fight or flight e.g.
-The heart beat increases hence increasing rate of circulation.
-Increases metabolic rate.
-Arterioles of the skin and digestive system constrict.
-In the liver, glycogen is converted into glucose.
-Skeletal muscles contract and relax which can allow movement.
-Breathing rate becomes faster and deeper.
-Fats are converted to fatty acids which are available in the blood for muscle contraction
NB over secretion can be brought about by growth of tumour in the medulla of adrenal glands. Symptoms are:
High blood pressure
Severe headache
Racing heart
Sweating
Faintness
The resultant effect is aging of major body organs such as kidney, heart and liver.
Comparison between endocrine and nervous system
Both provide a means of communication within the body of an organism.
Both involve transmission of a message triggered by a stimulus and a response
The target organs of hormones are like effector organs
Both involve chemical transmission.
Both bring about survival response.
Differences between endocrine and nervous system

– Endocrine system	– Nervous system
– Chemical substance to evoke reaction	– Nerve impulse to evoke response
– chemical transmitted through blood	– impulse only through nerve fibre
– response slow but affect several parts of the body	– responses quick, specific and localized
– effects are long lasting	– effects are rapid and short – lived
– responses take place involuntarily	– takes place voluntarily and involuntarily

Effects of Drug abuse on human Health.

Drug-Its any chemical substance which when taken into the body has psychological and physiological effects.
Drug abuse– It’s the indiscriminate use of drug with no regard to their side effects
Commonly abused drugs include:
Khat (miraa)
Nicotine
Cannabis Sativa
Alcohol
Prolonged abuse of drugs can cause addiction (drug dependence)
- Effects of Drug abuse
Depressed appetite and poor feeding habits leading to emaciation
Interference with absorption of vital vitamins such as vitamin K, E which may lead to sterility and blindness.
Lowers nervous coordination leading to loss of posture and balance. This decrease performance in sports and manual activity.
Irritation of the lungs and the respiratory tract leading to frequent coughing and infections.
May lead to cancer of the lungs, throat and that of urinary bladder.
May also cause stomach ulcers.
Damage too many tissues of the heart and liver leading to heart attack and liver cirrhosis respectively.
Interference of temperature regulation leading to excessive heat loss.
Damage caused to brain may lead to sleeplessness(insomnia) , loss of memory (amnesia), deliriums, hallucination and mental illness (madness)
In women, drug abuse may lead to poor foetal development and pregnancy complication.
Irreversible damage to vital body tissues and organs and may eventually lead to death.
Addicted persons have an impaired judgment which may predispose them to accidents and infections such as HIV\AIDS.
SENSE ORGANS
In mammals, the main organs of special sense are:
Eye for sight
Ear for hearing
Tongue for taste
Nose for smell
Skin for pressure, pain and temperature.
The Eye
The function is to receive light by which an animal perceives and distinguishes objects in its immediate environment.
The eye is located in a socket in the skull called orbit, which offers protection against physical damage.
Within the orbit there is a fatty layer lining which provides further protection as a shock absorber against mechanical damage.
In the socket the eye is suspended by sets of muscles which move the eye e.

Lateral rectus muscles- They move the eye left and right.
Superior and inferior rectos muscles –Move the eye up and down.
Oblique muscles – Steady the eye in it’s up and down movement.
In the front of the eyeball, there are two thin folds of skin, the eyelids which protect the eye.
From the edge of the eyelids, there are many hairs called eyelashes which protect the eye from entry of small particles.
Eye brows – they are raised portions of the skin above the eye, thickly covered with hair whose function is to prevent sweat and dust from entering the eye.
In land vertebrates there is a lachrymal (tear) gland that continuously secretes watery, saline & antiseptic fluid called tears.
The tears moisten the cornea and wash foreign particles out of the eye. The fluid drains through the lachrymal duct into the nose.
In mammals, the lachrymal gland is beneath the upper eyelid, while in other animals it is located beneath the lower eyelid.
In amphibians, birds, reptiles, some fish and some mammals, e.g. cat, there is a transparent membrane called nictitating membrane which is drawn across the eye to clean it.
The mammalian eye is spherical, fluid filled structure whose walls consist of three layers i.e.
Sclerotic (outer layer) / sclera
Choroid (middle layer
Retina (inner layer)
Sclera / sclerotic / outer layer
This is a white fibrous layer which protects the delicate inner part of the eyeball and helps in maintaining its shape.
The sclera forms cornea at the front of the eye.
- Cornea – it’s a transparent layer
It allows the light to enter the eye
It aids in reflecting the light entering the eye.
Conjunctiva:- It’s a protective thin transparent membrane covering the front portion at the cornea.
Choroid/ middle layer
It’s a dark-pigment, membranous layer. It has numerous blood vessels.-
Absorbs stray light hence prevents internal reflection within the eye.
Provides nourishment to the eye due to presence of numerous blood vessels.
At the front of the eye, the choroid extends and forms ciliary body and iris
Iris – it is a thin round sheet of muscular muscles (circular and radial) which controls the diameter of the pupil.
Its pigmented giving the eye its colour i.e. black, brown or blue
Pupil – it’s the opening in the iris which allows the light to enter the eye. In some vertebrates e.g. cats the pupil is narrow and slit – like while in most vertebrates it appears round.
Ciliary body – it’s an extension of choroid, iris and suspensory ligaments attached to it.
It contains circular and smooth muscles which contract and relax to alter the shape of the lens.
Ciliary body secretes the aqueous humour.
Lens – it is a transparent biconvex structure located immediately behind the pupil of the vertebrate eye
It’s held in position by suspensory ligaments which become tight or loose to alter the shape of the lens.
The lens divides the eyeball into anterior & posterior chambers
The anterior chamber i.e. the part behind the cornea is filled with a watery fluid called aqueous humour.
The posterior chamber i.e. the part between the lens & retina is filled with a denser, jelly – like transparent material called vitreous humour.
This fluid helps to maintain the spherical shape of the eyeball and refracts incoming light towards the retina.
Retina
It is the light sensitive layer composed of 3 regions i.e.
an outer pigmented region in contact with the choroid
a middle region of photoreceptors consisting of cones & rods.
An innermost region of neurones. These neurones run over the surface of the retina and join to form the optic nerve which transmits nerve impulses from the retina to the brain for interpretation.
Cones
Cones – they are densely packed together in a certain region of the retina called fovea or yellow spot. They contain the light sensitive pigment called iodopsin.
Iodopsin is adapted for:-
Bright light vision
Perception of fine details
For colour vision
When one looks at an object directly, light rays from it falls on the fovea. This enables the object to be observed in detail.
In the presence of light, iodopsin breaks down to iodine & opsin. Each cone has its own bipolar neurone which in turn links it with an optic nerve i.e. it lacks retinal convergence.
This property of the cones enables them to have high visual acuity (ability of the eye to distinguish objects clearly)
In higher vertebrates there are 3 types of cones cells which enable them to differentiate the different colours they perceive i.e. blue, green & red.
The trichromatic theory which suggests that the simultaneous stimulation of the 3 types of cones at different degrees brings about colour perception e.g.
- -Equal stimulation of red and green types of cones cells is perceived as yellow colour.
- -Equal stimulation of all 3 types of cones produces the colour sensation of white.
Rods
Rods contain a photochemical pigment called rhodopsin (visual purple) which perceives light of low intensity but is not sensitive to colour.
The rods have retinal convergence, therefore they have low visual acuity hence cannot distinguish fine details
Rhodopsin degenerates to opsin and retinal ( derivative of vitamin A) to bring about depolarization of the cell membrane, and then triggers off an impulse i.e. Rhodopsin light opsin + Retinal
The amount of rhodopsin in the eye is increased in the dark, raising the sensitivity of the rods to dim light.
Resynthesis of rhodopsin occurs slowly in the dark for continued photochemical reaction in the rods.
Rods are in greater concentration round the periphery of the retina and are absent in fovea centralis. Because of this, one can see an object better in dim light if he looks at it from the corner of the eye. This way, the image falls on the rods.
Diurnal animals (that operate during the day) like man have large number of cones in their retina.
Nocturnal animals (those that operate during the night like bats have large numbers of rods in their retina.
In the retina there is an area where the optic nerves enter the eyeball. This is called blind spot. This area has neither rods nor cones, so images from objects falling on the blind spot cannot be perceived.
Image formation and interpretation
Light from the object is reflected by cornea, aqueous humour and lens through the vitreous humour and focused on to the fovea centralis on the retina.
The image is recorded as real, inverted and small e.g.
The photoreceptor cell becomes stimulated and nerve impulse is generated and transmitted by the optic nerve to the cerebrum part of the brain for interpretation. In the brain the impulses are interpreted and the object appears real, upright and normal.
The images from the left eye are interpreted by the right cerebral hemisphere and those from the right eye by the left cerebral hemisphere.
Binocular vision / stereoscopic vision
Binocular vision refers to the ability of the right eye to provide a three dimensional view and a depth perception of an object under observation.
It’s seen in man and other primates all of whom have two eyes placed in front of the head. This way, both eyes can be focused on the same object. Each eye forms its own image of the object under the observation.
Both images are sent to the brain which combines them to give a single impression of the object, since each eye “sees” a slightly different aspect of the same object, a combination of the two images provides a 3 dimensional view and depth perception.
Binocular vision helps to accurately judge the distance as when monkeys leap on trees or when a man is driving.
If the two eyes are not well aligned or if the visual cortex is intoxicated e.g. by alcohol the object under observation appears double & blurred.
- NB Improper alignment of the eye can be demonstrated by looking at an object & pressing the eye to the side with a finger.
- Accommodation of the eye
This refers to the ability of the eye to focus for both far and near objects. It is accomplished through a change in the shape of the lens.
- Accommodation of a close object
The ciliary muscles contract thereby relaxing the tension on suspensory ligaments.
The curvature of the lens increases i.e. the close object are gently refracted by the lens focusing them onto the retina e.g.
- Accommodation of a distant object
The ciliary muscles relax thereby increasing the tension of the suspensory ligaments. This stretches the lens decreasing its curvature i.e. the lens become thinner. Light rays from a far object are less refracted and hence focused onto the retina e.g.
During the accommodation, the iris regulates the amount of light entering the eye.
In bright light, the circular muscles of the iris contract while the radial muscles relax and the pupil becomes smaller. This prevents damage of the retina by excessive light.
In dim light the radial muscles of the iris contract and the circular muscles relax, the pupil enlarges.
This allows in enough light to stimulate photoreceptors on the retina.
Defects of the eye.
- Short sightedness (Myopia)
It is also called near sighted and individuals with this defect have a longer than normal eye ball. Light rays from distant objects are focused at point in front of the retina.
As a result distant objects appear blurred.
This defect can be corrected by wearing glasses with concave (diverging) lenses. These bend light rays outwards before they reach the eyes enabling them to be focused on the retina.
- Long-sightedness (Hypermetropia)
Long sighted or far sighted individuals have a shorter than normal eyeball or weak lenses. Light rays from a near object are focused at a point behind the retina. As a result near objects appear blurred
Light rays from distant objects are focused normally on the retina e.g.
This defect can be corrected by wearing glasses with convex (converging) lenses. The lenses bend the light rays inwards before they reach the eyes enabling them to be focused on the retina
Astigmatism
This is a condition in which light rays from an object are brought to focus in different planes. This is caused by unequal curvature of the cornea or lens which produces unequal refraction of light entering the eye.
It’s corrected by wearing a special cylindrical lens in front of the eye which corrects the focus in the defective planes.
Squintedness
This is where the extrinsic muscle of the eye that control the turning of the eye ball do not co-ordinate above on stimulation.
The defect affects the paired rectus muscle which turn the eye up and down and lateral rectus which move the eye left and right.
In this condition, the eye ball face different direction hence focusing and accommodation are achieved with difficulty
Its difficult to correct this defect
Old sight (Presbyopia)
This is a condition in which the light rays from an object are brought to focus behind the retina, while rays from a distant object is sharply focused. This is by hardening (loss of elasticity)of the lens and weakening of the ciliary muscles due to old age
The defect is corrected by wearing a concave or convent lens or one pair of glasses with two different lenses called bifocal lenses.
Colorblindness
This is a genetic defect in which certain colours can not be distinguished by human beings and other animals.e.g Red-green colour blindness in which an individual is unable to distinguish between red and green.
The retina of the affected individual lacks cones with pigments that normally respond to red – green colors.
Currently there’s no cure for colorblindness.
Cataracts
This is an eye defect associated with old age. It may also be caused by eye injury due to a blow or by complications of diabetes mellitus.
The eye lens become cloudy or opaque hence blocking the transmission of light rays. The transparent protein fibres in the lens are denatured and coagulated forming the opaqueness in the lens.
The defect can be corrected surgically by replacing the diffective lens with a good one from a donor or an artificial lens.

THE HUMAN EAR
The mammalian ear performs two main functions i.e.
-Hearing
-Maintenance of balance
The ear can be divided into 3 main parts i.e.
The outer ear
It consists of;
-pinna
-External auditory meatus
The pinna
It’s a flap of skin and cartilage which partially covers the opening to external auditory meatus.Some animals e.g. cattle are able to rotate their pinna in order to locate the direction of sound.
It collects and concentrates sound waves into the external auditory meatus. It leads into external auditory meatus.
External auditory meatus
It’s a tube or passage that directs sound waves to the ear drum.
The tube is lined with hairs which help to trap solid particles that may enter the ear.
It’s also lined by wax secreting cells whose function is to secrete wax that traps dust and prevents entry of solid particles. Wax also maintains the flexibility of the ear drum.
The middle ear
It’s an air –filled cavity consisting of;
-Eardrum (Tympanic membrane)
-Ear ossicles
-Eustachian tube
-Oval window
-Round window
Eardrum (Tympanic membrane)
It’s taut but pliable like the skin of a drum which enables it to vibrate.
When it is hit by sound waves from outside, it vibrates and transforms sound waves into vibrations. It then transmits the vibrations to the Ear ossicles.
(ii) Ear ossicles
These are 3 bones namely;
-Malleus (hammer)
-Incus (anvil)
-Stapes (stirrup)
They are suspended by muscles i.e. tensor tympani and stapedius.These muscles also prevent excessive vibrations which could damage the inner delicate membraneous labyrinth.
The 3 Ear ossicles form a system of levers which amplifies and transmits the vibrations from Eardrum (Tympanic membrane) to the Oval window.
Eustachian tube
It’s a tube connecting the middle ear with the pharynx.
Its function is to equalize the air pressure between the middle ear and the outer ear to prevent the distortion of the Eardrum (Tympanic membrane).e.g. if you go higher up in an aeroplane, the atmospheric air pressure outside falls below that of the middle ear. This results in the Eardrum (Tympanic membrane) bulging outwards and the condition can be rectified by yawning or swallowing which opens the Eustachian tube to equalize the pressure on both sides of the Eardrum.
NB Eustachian tube can provide a passage for entry of pathogenic microbes from the pharynx to the middle ear causing ear infection.
Oval window
It’s a membrane that covers a small hole leading to the semi-circular canals
Round window
It’s a membrane that covers a small hole leading to the cochlea.
The Inner Ear
It’s a fluid-filled cavity. It consists of;
-Cochlea-Involved with hearing
-Vestibular apparatus – Composed of vestibule and semi-circular canals which are involved in
The cavities in the inner ear are filled with fluids called perilymph and endolymph. The fluids conduct sound vibrations transmitted from the middle ear to the cochlea for hearing.
In the vestibule and semi-circular canals the displacement of the fluids leads to the restoration of the body balance. The fluids absorb mechanical shock hence protect the delicate sensory structures.
Mechanism of hearing
Cochlea is spirally shaped tube consisting of a system of canals, membranes and sensory cells. The canals are filled with endolymph.and perilymph.The coiling of the cochlea offers a large surface area for attachment of the sensory cells responsible for hearing.
The pinna concentrates sound waves into the External auditory meatus.The sound waves strike the Eardrum and cause it to vibrate. The vibrations are transmitted to the ear ossicles in the middle ear.
The first ossicle, Malleus picks the vibrations, and then transmits to Incus then to stapes. The stapes passes the vibrations to the Oval window from where the vibrations are transmitted to the perilymph of the cochlea.
The 3 Ear ossicles are specially arranged to amplify the vibrations as they transmit the to the Oval window (amplifies 22 times)
In the cochlea the vibrations stimulate the sensory cells hairs to generate nerve impulses which are transmitted to the brain via the auditory nerve for interpretation. The intensity of stimulus transmitted to the brain enables the brain to interprete the impulses as sound of specific pitch and loudness.
Meanwhile the vibrations in the fluid of the inner ear are dissipated back into the middle ear through the Round window.
The direction of sound is detected accurately as a result of both ears functioning together. When sound waves come from the front, both ears pick the waves at the same time.
If the sound is from the sides, one ear will pick the waves earlier than the other. The time lapse of impulses to the brain allows for the determination of direction and distance.
Maintenance of body balance and posture
Balance is brought about by the semi-circular canals and the vestibule
Semi-circular canals
These are 3 tubular cavities containing endolymph.These canals lie mutually at right angles to each other and occupy the 3 planes of space. They contain receptors that respond to rotation of head in any of the 3 planes.
Each semi-circular canal has a swelling called ampulla at one end containing sensory cells.
The semi-circular canals maintain body posture in relation to movement of the head e.g. when one spins and then stops suddenly, one feels dizzy. This is because the fluid is still in motion and stimulating sensory cells in the ampulla. The movement of the fluid stimulates the sensory cells which trigger off nerve impulses which are transmitted via the auditory nerve to the brain for interpretation.
In the brain the information is relayed to the motor neourone to the muscles of the body to restore the correct posture.
- Vestibule
It consists of utriculus and succulus which contain sensory cells. They maintain posture and balance in relation to gravity.
When the body balance is shifted the fluid disturbs sensory cells. This triggers a nerve impulse to the brain via the auditory nerve. The brain interprets the impulse according to the position of the body in relation to gravity. The brain relays a nerve impulse through the motor neourone to the muscles of the body to restore the correct posture..
Defects of the Ear
Deafness-This is a hearing defect which makes an individual unable to perceive sound. There are two categories i.e.
Permanent deafness-This is due to damage of the cochlea or auditory nerve. It is caused by;
–Prolonged exposure to loud sounds.
–When the cochlea is sensitive to certain drugs e.g. some antibiotics
NB It’s difficult to correct
(ii) Partial deafness-It’s brought about by impairment of the structures that conduct vibrations to the cochlea e.g. ear drum and ear ossicles.
Ear ossicles. – Can be impaired due to abnormal growth of the connective tissue(fibrosis) in the middle ear or by calcification of ear canals.
Ear drum– Can be damaged by;
-Infection
-Physical blow
-Production of too much wax which hardens hence blocking the external auditory meatus.
-Partial deafness can be corrected by surgery or by using a hearing aid.

SUPPORT AND MOVEMENT IN PLANTS AND ANIMALS

Support-It’s the ability of organisms to bear their weight and maintain their body forms Movement- It’s the displacement of parts of the body of an organism e.g. growth Movements of plants and limbs of animals.
Locomotion- It’s Movement of the whole organism.
Necessity for support and Movement in plants
Movement enables plants to adjust to the environment e.g. growth of pollen tube to bring about fertilization.
Enables the plants to obtain resources from the environment like light, water and nutrients-Tropic and nastic Movements.
Enables plants to escape or avoid harmful stimuli such as high temperature.
Supportive tissues enable a plant to hold delicate organs like leaves and flowers where they can most efficiently carry out their functions.
Supportive tissues enable a plant to maintain its correct posture.
Enables plants to withstand external forces such as strong winds and animals climbing on them

Arrangement of tissues in stems

A stem plays an important role in support in a plant. This is due to the presence of support tissues which provide mechanical support. These tissues are;
Parenchyma
They are spherical or elongated. They form the major component of the cortex and pith.
When turgid they become tightly packed and rigid, thus enable a plant to attain an erect posture.
They provide the main support in many herbaceous plants such as tomatoes and Irish potatoes. However when such plants lose water on a hot dry day, the cells become flaccid and loosely packed. This causes the plants to droop a phenomenon called wilting.
(ii) Collenchyma cells
Not all stems develop collenchyma.It always occur towards the outside beneath the epidermis.
Their walls are thickened with cellulose especially at the corners to provide mechanical support.
Collenchyma cells contain living protoplasm and their walls are not lignified.
Collenchyma tissue is important in providing mechanical support in leaves, herbaceous plants and young woody plants.
(iii) Sclerenchyma
Their walls are thick and lignified. The Sclerenchyma cells are often long fibres in stems such as hibiscus.
Sclerenchyma consists of dead cells thickened by lignin. Lignin.is a complex polysaccharide.
It’s the main constituent of wood.
- Xylem vessels
They are thick walled tubes with lignin deposited unevenly in rings, spirals or patches on their walls.
Their main function is transport but because their walls are thick and lignified, they give strength and support to the stem.
They are found in angiosperms.
- Xylem tracheids

These are long cells with tapered ends .They are dead cells with thick unevenly lignified walls and help to strengthen and support the stem.
Based on the nature and distribution of the strengthening tissues, there are two types of stems i.e.
- Herbaceous stems
The tissue is relatively soft and easily crushed. Plants with this kind of stem are usually small and do not grow very tall.
Mechanical strength in such stems is directly related to the turgor pressure of the living cells of which it is composed.
Some herbaceous plants are known to obtain support by twinning round other plants e.g. passion fruit stems and morning glory e.g.
Others support themselves by use of tendrils e.g. Pumpkins
(ii) Woody stems
They have support tissues whose cells have stiff, thickened or lignified walls. Therefore they are referred to as strengthening tissues. These tissues are; collenchyma, sclerenchyma, xylem vessels and tracheids. Even when completely dry, these cells remain strong and maintain their shape.
They develop thick, strong trunks and branches and those plants live for many years.
Stems of woody plants grow in height and in diameter from year to year.
Stems of woody plants when young exhibit herbaceous characteristics in terms of support but as they mature they undergo secondary growth that leads to development of elaborate tissues e.g. bark covering their trunks.
Activity1; To observe wilting in plants
- Materials
-Herbaceous plant-Irish potatoes/Sonchus
-Woody shrub-Sodom apple/Mexican marigold
- Procedure
Uproot a young herbaceous plant and a woody shrub within the same span of time.
Lace the two plants on the laboratory bench for 30 minutes.
Observe the appearance of stem and leaves of;
herbaceous plant
Woody shrub
Account for the appearance of the shoot in the two plants above.
Support and Movement in Animals
Animals have a firm and rigid framework for support known as a skeleton.
Functions of skeleton.
Supports the weight of the animal’s body.
Gives the body its shape.
Provides surface for attachment of body muscles to facilitate movement
Internal organs are attached onto the framework or suspended from it.
-Animals move from place to place to;
Search for food.
Escape from predators or hostile environment.
Look for mates and breeding grounds.

Types of skeleton

Hydrostatic skeleton.
Exoskeleton
Endoskeleton
- Exoskeleton
It’s a characteristic of members of the phylum Arthropoda and is made up of a substance called chitin. Chitin is secreted by epidermal cells and hardens on secretion.
This Exoskeleton supports and protects inner delicate tissues.
It’s waterproof and therefore prevents excessive loss of water from the body tissues.
Exoskeleton provides a surface for attachment of body muscles which is essential for movement.
Chitin is not evenly distributed i.e. it is thin at the joints to allow for efficient movement.
NB Exoskeleton limits growth. To overcome this limitation it is therefore periodically shed, a process called moulting (ecdysis).
Endoskeleton
It is a characteristic feature of all vertebrates.
It’s made up of living tissues i.e. cartilage or bones, hence these tissues grow steadily within the animal.
- Functions
Supports the animal’s body weight.
Gives the body its shape.
Protects inner delicate organs e.g. heart, lungs, brain from mechanical injury.
It provides surface for attachment of body muscles when they contract or relax to bring about movement.
The long and the short bones of the skeleton produce the blood cells.
Calcium and phosphate deposits on the bones cause hardening of bone tissue therefore bone acts as a reservoir of Calcium and phosphate ions in the body.
- Locomotion in a finned fish –Tilapia
-The finned fish are adapted for movement. In the water in the following ways;
They have a streamlined body which reduces resistance against movement and enables it to cut through the water easily.
It has an inflexible head that enables it to maintain forward thrust.
The scales of the fish overlap and are pointed backwards to allow the water to pass over the fish easily without any obstructions.
The fish also secretes mucus which covers the body and this reduces friction during movement.
The fish has a flexible backbone on which segments of muscle blocks (myotomes) are attached.
The fish has strong tail muscles which contract and relax to bring about undulating movements.
Some fish posses a swim bladder between the vertebral column and the gut. It provides fish with the buoyancy and also helps the fish to adjust its vertical position in relation to depth in water.
The fish posses a lateral line system along the length of their body that enables it to detect vibrations and changes of pressure in water thus enabling the fish to respond suitably.
The tail has a large surface area which increases the amount of water that is displaced resulting in an increase in the forward thrust.
They have two types of finse.;
Paired fins-Pectoral and pelvic fins
(ii)Unpaired fins-Dorsal, ventral and caudal fins
- Paired fins- Pectoral and pelvic fins
-They have the following functions;
- Maintaining balance
- Braking-When the Paired fins are extended rapidly forward at right angles to the body.
- Changing direction-Each Pectoral fin can be used independently of its opposite member hence act as pivots around which the fish can turn rapidly.
Control pitching of the fish i.e. the tendency of the fish to plunge down head first e.g.
- Unpaired fins- caudal, Dorsal ,and ventral (anal)fins
-They have the following functions;
Reduce rolling i.e. fish rolling to one side.
Reduce yawing i.e. lateral deflection of the body.
NB Caudal fin propels the fish forward and steers fish while in motion.
- Activity 1; To examine external features of a finned fish
- Requirements
Freshly killed finned fish in a tray.
A pair of forceps

Procedure

-Examine the fish provided and identify the following features;
Scales
Fins-caudal, ventral, Dorsal, pelvic and Pectoral fins
Note the shape of the fish’s body.
Place the fish to lie on its side on the bench. Stroke the fish from the head to the tail using your fingers and from the tail to the head. Record your observation in relation to the arrangement of the scales.
Draw and label the fish.
- Activity 2; To calculate the tail power of fish
Requirements
Freshly killed finned fish in a tray
Means of measuring(in millimeters)
Procedure
Obtain a Freshly killed finned fish.
Measure its body length from the mouth to the tail tip.
Measure the length from the tail tip to the anus.
Calculate the percentage length of the tail to the rest of the body.
Tail power is given by;

Tail power=Length from the tail tip to the anus ×100
- Length from the mouth to the tail tip

Support and Movement in mammals

In mammals the bones and muscle work together to bring support and movement.
The skeleton system in mammals is divided into two parts i.e.;
- Axial skeleton
- Appendicular skeleton
Axial skeleton
This consists of the following;
Skull
It consists;
Cranium
It encloses and protects the brain. There are perforations on the cranium to allow blood vessels and nerves to pass to and from the brain.
Lower jaw (mandible) and upper jaw (maxilla)
It articulates (forms a joint) with the upper jaw (maxilla) through a hinge joint.
At the posterior end of the cranium are two smooth rounded protuberances, the occipital condyles, which articulate with the atlas vertebra to form a joint which permits the nodding of the head.
X
Ribcage
It encloses the thoracic cavity protecting delicate organs e.g. heart and lungs.
The cage is made of ribs articulating with vertebral column to the back and the sternum to the front. At the articulating points, the ribs have cartilage.
Sternum
It supports the ribs and protects the organs in the thoracic cavity.
In flying vertebrates the sternum is very prominent and often modified to form a keel which gives a large surface area for attachment of pectoral muscle (flight muscle)
NB At the lower end, the sternum and ribcage offer surfaces for attachment of muscles of the back and the abdomen.
(d) Vertebral column
The Vertebral column consists of bones called vertebrae. The number of vertebrae varies from species to species e.g. in human there are 33 vertebrae.
The vertebrae are separated from each other by cartilage called intervertebral disc which has various functions i.e.
Acts as cushion that absorbs shock and reduces friction.
It makes Vertebral column flexible by allowing for a certain degree of movement between the vertebrae.
The Vertebral column has 5 types of vertebrae
- -Cervical vertebrae
- -Thoracic vertebrae
- -Lumbar vertebrae
- -Sacral vertebrae
- -Caudal vertebrae
The vertebrae have got common basic plan e.g.

The parts of a vertebra are;
- Centrum-It’s a solid structure of the vertebra. It supports the weight of the vertebrae’
- Transverse process-Are lateral in relation to Centrum while the neural spine is dorsal to the Centrum.
These two are projections which offer surfaces for muscle and ligament attachment.
- Neural canal-It’s a centrally running passage for the spinal cord.
- Neural arch-It’s an arch of bones which arises from the Centrum. Together with Centrum they protect the spinal cord.
The vertebrae articulate with each other anteriorly and posteriorly by facets called zygapophysis.
At the anterior (front) and posterior (back) of each vertebra is a pair of smooth facets for articulation of successive vertebrae.
Facets at the anterior parts are called prezygapophyses. They face upwards and inwards.
Facets at the posterior part are called postzygapophyses. They face downwards and outwards.
- Cervical vertebrae
They are found in the neck region. There are 7 cervical vertebrae.
All Cervical vertebrae have vertebraterial canals in the transverse process for the passage of vertebral artery and vertebral nerves.
The first two cervical vertebrae called atlas and axis are different from other cervical vertebrae.
- Atlas
Has a small neural spine.
It has no Centrum.
Neural canal is wide for the passage of the large spinal cord in the neck region.
Their transverse processes are broad and wing-like offering a large surface area for attachment of neck muscles.
It has broad facets for articulation with condyles of the skull. This forms a joint which allows for up and down movement of the head (nodding).

Axis

It’s the second cervical vertebra on the neck region.
The Centrum is broad and projects in front to form an odontoid process/peg. This forms a peg which fits into the ventral side of the neural canal of the atlas.
The joint between the atlas and axis allows turning movement of the head (rotatory movements).
It has broad neural spine.
Has wide neural canal.
Has wing-like transverse process.
- -The other 5 cervical vertebrae posses;
Broad and branched transverse process. These offer a large surface area for attachment of neck muscles
Short neural spine
Wide neural canals and wide Centrum.
They posses the prezygapophyses and postzygapophyses.
- (b) Thoracic vertebrae
They are found in the thoracic region articulating with the ribs. In man they are 12 in number
They have a long neural spine which offers a large surface area for attachment of back muscles.
Centrum is large.
Short transverse process.
The ribs articulate with the vertebrae at two points i.e. capitulum and tuberculum.The tuberculum facet on each transverse process articulates with tuberculum of the rib while the capitular demi-facets on the Centrum articulate on the capitulum of the rib.

Lumbar vertebrae

They are found in the lumbar region of the body. In man there are 5 vertebrae.
They have large and broad Centrum to offer support.
They have broad and long transverse process projecting forwards and downwards from the Centrum for muscle attachment.
Have broad neural spine for muscle attachment.
Infront and on either side of the neural spine are two projections called metapophyses.
Also projecting dorsally near the transverse process are anapophyses.
In some animals e.g. rabbits another projection the hypapophysis arises ventrally to the Centrum. All these projections offer additional surfaces for the attachment of abdominal muscles.

NB The vertebrae in the lumbar region are adapted to support the weight of the body and to withstand strains of movement.
Sacral vertebrae
They are situated in the sacral region. Man has 5 while rabbits have 4.
They have large and broad Centrum to offer support.
Neural canal is narrow.
Neural spine is short.
The first anterior sacral vertebra is large with wing-like transverse process which is fused to the pelvic girdle.
The transverse processes of the remaining vertebrae are not attached. They all offer a large surface area for attachment of back muscles.
All the sacral vertebrae are fused to form a rigid structure the sacrum. This makes sacrum strong and firm to bear the body weight and spread it to the legs through the pelvic girdle.

Caudal vertebrae.
They are found in the tail region. The number depends on the size of the tail. In man where the tail is vestigial they are 4 and are fused to form a coccyx.
Neural spines and zygapophyses are short.
Neural canal and neural arch are absent hence the entire bone is Centrum.
- Appendicular Skeleton
It consists of the girdles and limbs attached to them. The girdles are the pectoral girdle on the anterior side and pelvic girdle to the posterior.
The limbs to the anterior part of the body are fore-limbs and those to the posterior are hind-limbs.
The limbs of all mammals are constructed in the same basic plan with 5 digits (fingers and toes) in each limb. This is called pentadactyl limb plan e.g.
- Bones of fore-limbs
- Pectoral girdle
This is made up of two halves each of which consists of 3 bones i.e.
-Scapula
-Coracoid process
-Clavicle
These bones are attached to the upper part of the vertebral column. The two halves are not fused but are attached firmly by muscles.
Scapula (Shoulder blade)
It’s a flat, triangular-shaped bone. At its apex is a concave cavity or depression, called glenoid cavity which articulates with the head of humerus to form the ball and socket joint.
A spine runs along the outer surface of the scapula and at its free end, there are two projections i.e. acromion and metacromion which are both for muscle attachment.
- Clavicle (collar bone)
It articulates on one end with acromion process and the other with sternum. It’s for muscle attachment and aids in movement of arms.
Humerus
This is the bone found in the upper arm. Its head articulates with scapula at the glenoid cavity of the pectoral girdle where it forms ball and socket joint.
Near the head are two roughened projections i.e. the greater and lesser tuberosities. These extend into a shaft which provides surface for muscle attachment.
Between the tuberosities, is a groove called bicipital groove. It is along this groove that the tendons of the biceps muscles pass.
At the lower end is the trochlea which articulates with the forearm to form a hinge joint at the elbow.
- Ulna and Radius
These are two bones found in the forearm. The radius is found on the side of the thumb.
The ulna is on the side of the small finger and has a projection called olecranon process. This has a sigmoid notch which articulates with the humerus forming a hinge joint.
- Functions of olecranon process
Offers a large surface for attachment of tendons, ligaments and muscles
Prevents overstretching of the lower arm
Limits movement at the joint (acts as stopper).
Limits movement of radius and ulna+
- Carpals, metacarpals and phalanges
Carpals- They are small bones found in the wrist (wrist bones). They are nine (9) in the
Metacarpals (Hand bones) – They form the skeleton of the hand and are 5 in number.
Phalanges (Finger bones) –Each digit has 3 bones called phalanges singular phalanx except the thumb that has two.

- NB In cattle and goats the fore limbs has two digits while in the horse there is only one digit.
- Bones of Hind limb
These are;
The pelvic girdle
It consists of two halves fused at the pubic symphysis. Each half is made of 3 fused bones i.e.
- -Ilium
- -Ischium
- -Pubis
Each half has a cup-shaped cavity, the acetabulum. This articulates with the head of the femur to form a ball and socket joint. Dorsally, the ilium articulates with the sacrum.
The ilium provides a large surface to which thigh muscles are attached.
Has sacral facet that attaches it to the transverse process of the 1^st sacral vertebra
Between Ischium and pubis is a hole called orbiturator foramen. This is an aperture through which blood vessels, nerves and muscles pass. This design is an adaptation to reduce the weight of the pelvic girdle and hence lighten the load to be supported by the hind limb.
The pubic symphysis is composed of flexible cartilage which permits the widening of the females girdles when giving birth.
The ilium, Ischium and pubis are fused to form the innominate bone.

NB The size of the pubic cavity is important in females in regard to childbirth. A hormone called relaxin causes the relaxation of the pubis symphysis during birth thus expanding the size of the pelvic cavity.
Femur

It’s a long bone found between the hip and the knee. The head of femur fits into the acetabulum forming the hip joint.
At the tip of the shaft are the greater and lesser trochanters which are extensions for muscle attachment.
The shaft of the femur leads to the lower end with expanded and rounded knobs called condyles.
The condyles articulate with the patella (knee cap). They also articulate with tibia to form hinge joint at the knee.
The tibia and fibula form the skeleton of the lower hind limb.
The heel has 7 small bones in man called tarsals (ankle bones). These articulate with the tibia and fibula on the upper end and with the metatarsals (foot bones) on the lower end.
The metatarsals are 5 in number and form the skeleton of the foot. In man, there are 5 digits (toes) on the foot, 4 in rabbits, 2 in the cow and 1 in the horse. Each digit has 3 phalanges (toe bones) except the big toe that has two.

- JOINTS
A joint is a connection between two or more bones. There are various types of joints e.g.
- Immovable joints
g. Fused bones in the skull and the pelvic girdle.
Gliding joints e.g. at the wrist, ankle and between vertebrae in the vertebral column.
- Movable joints
They are found at various points of the appendages. These joints are characterized by bones covered with cartilages at the ends and bones being held together by tough ligaments.
The joint area is filled with a lubricating synovial fluid and is also called synovial joints.
Synovial joints are of two types.
- Ball and socket joints
This is a type of joint with two bones, one with a round head and the other one with a depression or a cavity into which the head of the first bone fits and moves freely.
In this joint, movement is possible in all directions.
- Examples; Hip and shoulder joints
This joint allow the limbs to rotate through 360°, however, they cannot bear very heavy loads.

- Hinge joint
The depressions in one bone allow the smooth condyles of another bone to fit and articulate to allow movement in one direction.
The maximum stretch of the limb at this joint is 180º. They are found at the elbow, knee and phalanges.
- Ulna – humerus hinge joint

Movement of a joint

At a movable joint the bones are held together by an inelastic tissue called Ligaments restrain movement of the bones thus preventing dislocation.
At the joint, muscles are attached to the bones by an inelastic tissue called
A muscle is attached to two points, the origin on an immovable bone and insertion on a movable bone.
Muscles which operate joints are in pairs and are antagonistic.
A muscle may bring about bending at a joint. This type of a muscle is called flexor muscle while the other which straightens the limb is the extensor muscle.
- Movement at the hinge joint of the elbow
In the arm there are two antagonistic muscles i.e. biceps and triceps.
The biceps (flexor muscles) flex the arm while the contraction of triceps (extensor muscles) extends the arm. The biceps contract and triceps relax. This pulls the ulna – radius upwards hence bending the arm.
While the triceps contract the biceps relax thus straightening ulna – radius leading to extension of the arm. e.g.
During this movement of the arm, the hinge joint at the elbow serves as the fulcrum with the biceps muscles providing the effort to lift the load at forearm.
Structure and function of muscles
Muscles are tissues specialized for contraction.
There are 3 types of muscles i.e.
- Skeletal muscle
- Smooth muscle
- Cardiac muscle
- Skeletal/striated muscle
They are attached to the bones/skeleton and are responsible for locomotion and other voluntary movements.
They are innervated by the voluntary part of the nervous system therefore known as voluntary muscles.
Its fibres have stripes running across them hence they are also known as striated or striped muscle. They contract and fatigue rapidly.
They are made up of long cylindrical cells with multiple nuclei (multinucleated).
Each fibre contains many myofibrils running parallel to each other.
A skeleton muscle is made up of a bundle of long fibres running the whole length of the muscle. The covering of a muscle fibre is called a sarcolemma.
The myofibrils are composed of two proteins strands i.e. actin and myosin.
Structure of striated muscle
The functional unit of the muscle is the myofibril which has the ability to contract. Muscles require large amounts of energy to contract this energy is provided by the numerous mitochondria present in the sarcoplasm.
Sodium ions and calcium ions are also necessary for muscle contraction.
The force created by contraction is transmitted onto the skeleton in the same magnitude by the tendon. This brings about the appropriate movement of the skeleton.
- Smooth/visceral muscles
They are found in the walls of tubular body structures e.g. digestive tract, blood vessels, urinary tract, reproductive tract, respiratory tract etc.
They are made of long spindle – shaped cells with a single nucleus.
They contain myofibrils enclosed by plasma membrane.
They lack cross striations hence referred to as smooth muscles.
They are innervated by the autonomic nervous system and can therefore not be contracted at will, hence they are also called involuntary muscles. They are capable of contracting slowly and fatigue slowly unlike skeletal muscles.
- (c) Cardiac muscle
This is the heart muscle. Each muscle fibre consists of short cells with centrally placed nuclei and numerous striated myofibrils.
The ends of the cells are marked by thickened region called intercalated discs. These form bridges between fibres hence transmit impulses rapidly throughout the heart.
The contractions of the heart muscle are generated from within the heart itself without nervous stimulation. Therefore heart muscle is said to be myogenic.
Cardiac muscle is capable of continuous rhythmic contractions without fatigue throughout the life of the mammal.
They have more mitochondria than skeletal muscle to sustain the energy demands.

Biology Form 4 Best Notes for all Topics

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