GENETICS

This is the study of inheritance and variation.

Terms used in genetics

1. Inheritance; transmission of characteristics from the parents to the offsprings
2. Variation; possession of characteristics different from those of the parents and other offsprings.
3. DNA; De-oxyribonucleic acid
4. RNA; Ribonucleic acid
5. Monohybrid inheritance; inheritance of one characteristic controlled by one pair of hereditary factors e.g. Tallness
6. Dihybrid inheritance; inheritance of two characteristics at the same time e.g. colour and Texture/shape in the garden pea plant
7. Dominance; ability of a trait to only express itself
8. Recessiveness; a trait that only expresses itself when in homozygous state.
9. Heterozygosity; presence of two dissimilar members of an allele e.g. Rr, Tt etc.
10. Homozygosity; presence of two similar of an allele e.g. TT, RR, tt, rr etc.
11. Allele; one pair of genes which occupy corresponding loci/positions in homologous chromosomes
12. Phenotype– the physical appearance of an individual or organism. It’s influenced by the genotype and environment.
13. Genotype– the genetic constitution of an organism. Its purely genetical.
14. F1 generation (first filial generation) – are the offsprings that represent the first generation of organisms or individuals under study.
15. F2 generation (second filial generation) – these are offsprings obtained after self crossing the F1 gen.
16. Incomplete dominance/ co-dominance– a condition where no allele is dominant over the other. The phenotype of the offspring is intermediate between that of the parents.
17. Multiple allelism– are characteristics determined by more than two variant forms of a single gene e.g. inheritance of the Blood groups in man (ABO)
18. Test cross / back cross– it’s a crossing involving a homozygous recessive to determine the genotype of an organism.
19. Mutation- these are spontaneous changes in the individual’s genetic makeup.

Concepts of Variation

Variation refers to observable differences among living organisms.

Types of variation

1. Discontinuous variation – in this type of variation, there are distinct and definite groups of individuals with no intermediate forms. E.g.
   1. sex either male or female,
   1. blood groups- one can only belong to one of the four blood groups A,B,AB,O
   1. ability to role the tongue
   1. Presence of long hair in the nose and on the ear pinna.
   1. Presence of a free or attached ear lobe.

All these traits are controlled by one or two major genes. These traits are not influenced by the environment

• Continuous variation – this variation has a wide range of differences for the same characteristic from one extreme to the other e.g.
  • Height
  • Skin colour/pigmentation
  • Weight
  • Length of internodes
  • Number of leaves, fruits on a tree etc
  • Finger prints

When these traits are plotted on a graph a normal distribution curve is obtained.

Number of individuals

 

A normal distribution curve of heights

This type of variation is brought about by the interaction of both the genetic environmental factors.

E.g. a plant with genes for tallness may fail to grow tall due to climate and poor soils.

Practical Activity 1

• Tongue rolling
• Finger prints
• Height

Causes of variation

1. Gamete formation – during gamete formation two processes contribute to variation. These are
   1. Independent assortment– during metaphase I of meiotic division, homologous chromosomes come together in pairs and segregate into daughter cells independently of each other. This independent assortment produces a variety of different gametes. The total number of combinations is given by 2ⁿ  where ‘n’ is the haploid number of chromosomes. In man n = 23 hence 2ⁿ = 2²³ which is equal to 8,388,608.
   1. Crossing over during the prophase I of meiotic cell division. I.e. when homologous chromosomes break and rejoin at certain points called chiasmata.
   1. Fertilization – during fertilization parental genes ca come together in different combinations. Therefore desirable and undesirable qualities of parents can be combined in the offsprings.
   1. Mutation– these are spontaneous changes in the genetic makeup of an organism. Mutation brings about changes in the living organisms.

The Chromosome

Each chromosome is made up of two parallel strands called chromatids. Each pair of chromatids is connected at a point by the centromere. Chromosomes contain the hereditary material called the genes. All cells including the sperms and ova have chromosomes.  Chromosomes are present in the nucleus and are only visible under the microscope during cell division – mitosis and meiosis. There is a definite number of chromosomes in each cell for every species of animal or plant.

E.g. in man somatic cells (body cells) have 46 chromosomes while the sex cells/gametes have 23 chromosomes. During fertilization fusion of the sperm- 23 chromosomes and the ova -23 chromosomes restores the 46 chromosomes to form a diploid zygote.

Chromosomal numbers in different organisms

Organism	Number of chromosomes
Somatic cells – 2n	Gametes – n
Sheep (Ovis auries)	56	28
Cow (Bos Taurus)	60	30
Fruit-fly (Drosophila melanogaster)	8	4
Maize (Zea mays)	20	10
Wheat(Triticum vulgare)	14	7
Tobacco (Nicotiana tabacum)	12	6
Man ( Homo sapiens)	46	23

Practical Activity 2

• Chromosomal behaviour during mitosis
• Chromosomal behaviour in meiosis

GENES AND DNA

Genes occupy specific positions on the chromosomes called the gene loci( gene locus)

The gene is a chemical in nature. The genes are in the form of a nucleic acid molecule called De-oxyribonucleic acid (DNA). In 1953 two Biologists Francis Crick and James Watson worked out the structure of the DNA. DNA was found to be composed of three different components;

1. A five carbon sugar-pentose
   1. phosphate molecule
   1. nitrogenous base

There are four types of the nitrogenous base;

• Adenine – N
• Guanine – G
• Thymine- G
• Cytosine – C

A combination the pentose sugar, a phosphate molecule and a nitrogenous base forms a nucleotide.

I.e.

DNA structure contains several nucleotides fused together to form long chains called DNA strands. Two parallel strands twist on one another forming a double helix structure. Adenine always combines with Thymine and Cytosine with Guanine.

Diagram

Role of DNA

• Stores genetic information in a coded form
• Enables 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

• During cell division both daughter cells arising from mitotic division have the same genetic constitution as the parent cell. DNA in the parent cell must therefore duplicate accurately before the cell divides. The process through which a DNA molecule forms an exact Replica is called DNA replication.
• The two strands forming the double helix separates like a zipper. Each parallel strand becomes a template that specifies the base sequence of a new complimentary strand. Through the action of replicating enzymes, free nucleotides take up positions along the template strands.
• The specificity of the base pairing ensures that only complimentary bases link together with those on the template strands. I.e. G-C and A-T.
• Covalent bonds are formed between the nucleotides resulting in the formation of a new DNA strand.
• The template and the new DNA strand the undergo coiling to form a double helix. In this way, two identical DNA molecules are formed from the original single molecule.
• Each of the new DNA molecules gets incorporated into one of the two nuclei formed just prior to the separation of the daughter cells.

Diagram

Role of the DNA in protein synthesis

The sequence of bases along the DNA strand acts as the alphabet and determines the sequence of amino acids when they join to form a polypeptide chain. Protein synthesis takes place in ribosome’s found in the cytoplasm. Since the DNA molecules are confined in the nucleus, there has to be a way of communicating the DNA information to the ribosome’s where actual protein synthesis occurs.

The cell therefore has a special molecule called the Ribonucleic acid (RNA). Its role is to carry genetic information from the DNA to the site of protein synthesis in the cytoplasm. It’s referred to as messenger RNA (mRNA). RNA is formed from the DNA strands.

During formation of the mRNA a section of the DNA strands acts as the template strand. The double helix of the DNA unzips and free nucleotides align themselves opposite the template. The base sequence of the template strand is copied onto a new strand.

In RNA, Thymine is replaced by another base called Uracil (U)

The transfer of DNA sequence on the mRNA strand is referred to as Transcription.

Diagram

After its formation the mRNA leaves the nucleus with instructions from the DNA about the kind of protein to be synthesised by the cell. This information is in the form of base triplets known as Codons which code for a particular amino acid of a protein molecule e.g.

• AAA-phenylalanine
• TTT-lysine
• CAA- valine
• CTA- aspartic acid

Differences between DNA and RNA

DNA	RNA
Has De-ox ribose sugar	Has ribose sugar
Double stranded	Single stranded
Confined in the nucleus	Found in nucleus and cytoplasm
Have organic bases as cytosine, guanine, adenine and thymine.	Has organic bases as cytosine, guanine, adenine and uracil

THE FIRST LAW OF HEREDITY

MENDEL’S EXPERIMENTS

An Austrian monk known as 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. The characteristics include:

• Height of the stems- tall or dwarf
• Texture of the seed coat- smooth or wrinkled
• Colour of the seeds- yellow or green
• Texture of the ponds
• Colour of the flowers- white or  purple
• Position of the flower- axial or terminal

He selected a group of dwarf plants and self pollinated them by dusting mature pollen grains onto the stigmas of the same plant. He collected the resulting seeds and planted them. He noted that these seeds grew into dwarf plants only. He repeated the experiment for several generations and obtained the same results.

In another experiment, Mendel selected tall plants and self-pollinated them. He planted the resulting seeds and observed that they grew into a mixture of tall and dwarf plants. He took seeds from the tall offsprings only and repeated the experiment for many generations until he obtained only tall plants.

This way he was able to obtain a pure line of tall garden peas and a pure line of dwarf garden peas.

He then cross-pollinated pure bred tall garden pea s with the pure bred dwarf variety. He planted the resulting seeds and he observed that all the offsprings were tall plants.

Diagram

He further crossed two of these tall offsprings and planted the resulting seeds. Mendel observed that this second generation consisted of a mixture of tall and dwarf plants. After counting these plants he noted that the ratio of tall to dwarf plants was approximately 3:1 respectively. He observed that this ratio was always obtained when crosses were made between the non-pure breeds of tall plants.

Diagrams

Mendel concluded that the traits of an organism are determined by hereditary factors which occur in pairs. Only one of pair of such factors can be represented in a single gamete. This later became Mendel’s First Law, The Law of Segregation

At this time Mendel had no idea of genes and so he called them factors. He postulated that these factors are found on the chromosomes and are passed from the parents to the offsprings via gametes.

Reasons behind Mendel’s success

1. He used favourable materials i.e. the garden pea plant which is normally self pollinated. This made it easy for him to employ cross pollination at will.
2. the pea plant he used had several contrasting traits
3. His study was focused on particular traits while those before him had been attempting to determine wholesome heredity of each organism.
4. He kept accurate data on all his experiments and fro the analysis of this data he was able to formulate definite hypothesis.

Mendel chose the garden pea plant because of the following reasons

• Plant had many contrasting traits e.g. flower colours,  seed coat texture, length of the stems etc.
  • Plant is normally self pollinated but cross pollination can be employed t will.
  • Plant matures relatively fast
  • Plant produces many seeds that can be planted to produce many offsprings

Monohybrid Inheritance

This is the inheritance of one trait like height in the garden pea plant that is controlled by a single pair of hereditary factors (genes) contributed by both parents. Genes occur in pairs on chromosomes and such gene pairs are known as alleles.

The genetic constitution of an organism is called the Genotype while the physical appearance is known as the Phenotype.

The genotype of an organism is represented using paired letter symbols. Capital letters represents the dominant gene while small letters represent the recessive gene.

Genetic Cross

Components of a genetic cross

• Parental phenotypes
• The parental genotype –the crossing X should be shown here.
• The gametes and should be circled.
• The fusion process or fertilization.
• The filial generation genotypes

NB. The conventional symbol for male is ♂ and that of female is ♀

Example 1

During gamete formation in the dwarf plat, each gene in the pair segregates into different gametes. When the female and male fuse during fertilization, the offspring produced contain the same number of genes as in each parent. The inheritance of dwarf ness in the pea plant can be illustrated diagrammatically by the following genetic cross

Diagram

Example 2

Similarly the pair of genes in the pure breed tall plants will segregate into different gametes during gametogenesis. When self fertilised the resulting seeds will have half the number of genes from each parent i.e.

Diagram

Example 3

When the purebred tall plant is crossed with dwarf plants, the resulting seeds grow into tall plants only. These offsprings represent the first generation (F1 gen)

Diagram.

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 a reciprocal cross. The F1 results will be the same for either cross.

Diagram

Example 4

When the F1 offsprings are self pollinated, they produce offsprings which that grow into a mixture of tall and dwarf plants. These offsprings are known as the F2 gen.

Diagram

A Punnet Square can also be used to work out genetic crosses e.g.

Parental phenotype   tall          tall

Parental genotype     Tt     X   Tt

Gametes                T t            T  t

♀ ♂	T	t
T	TT	Tt
t	Tt	Tt

When the allelic genes are identical, as in TT and tt, the condition is known as homozygous. An individual     

with such a condition is known as a homozygote.

When the allelic genes are not identical as in Tt, the condition is referred to as heterozygous. An individual with such a genotype is referred to as a heterozygote. An individual with genotype Tt, will be physically tall because the gene T is dominant over t.  The allele t is recessive.

A dominant gene expresses itself in both the homozygous (TT) and heterozygous (Tt) states while a recessive gene only expresses itself I its homozygous state (tt). TT is therefore referred to as homozygous dominant and tt is homozygous recessive.

The ratio 3 tall: 1 dwarf, in the F2 gen is characteristic of monohybrid inheritance where one gene is completely dominant over the other. This is referred to as complete dominance.

The monohybrid crosses are based on Mendel’s first law, The law of Segregation which states the characteristics of an organism are determined by internal factors which occurs in pairs. Only one of a pair of such factors can be represented in a single gamete.

Diagram – the process of segregation

Ratios and Probability

The 3:1 ratio in monohybrid inheritance can be represented in the form of probability. When a large number of heterozygous garden pea plants are selfed, the probability of getting tall plants is ¾ or 75% and that of dwarf will be ¼ or 25%.

Diagram

NB. The inheritance of characteristics involves probability. The chance that a particular gamete will fuse with another is a random occurrence, in genetics this done by showing all possible fusions.

Practical Activity 3 and 4

• Tossing a coin
• To demonstrate random fusion of gametes in monohybrid inheritance.

Similar monohybrid inheritance results as those of Mendel have been obtained by using the fruit fly (Drosophila melanogaster) the insect has many observable characteristics that are contrasting such as,

• Wing length – long wing dominant over vestigial wing
• Eye colour – red eyes dominant over white eyes
• Size of the abdomen – broad abdomen dominant over narrow abdomen
• Body colour – grey body colour dominant over black body colour.

Assignment

Using appropriate letters work out the following crosses with respect to the fruit fly

• Cross between  a purebred long winged and a vestigial winged
• Cross between two long winged heterozygotes
• Cross between a red eyed heterozygote and a white eyed fruit fly.

The fruit fly is suitable for genetic study because of the following reasons.

1. The female lays very many eggs hence increasing the sample size for study.
   1. Have many observable characteristics that are distinct and contrasting.
   1. It is easily bred in the laboratory with minimum requirements.
   1. It has a short generation time 10-14 days. Therefore many generations can be studied in a short period of time.
   1. Offsprings can be crossed with their parents at will (backcrossing)
   1. Flies are safe to handle because they do not transmit any known human diseases.

Study Question 10

Practical Activity 5

• Breeding fruit flies.

Incomplete Dominance (Co-dominance)

In Mendel’s experiments with garden pea p[plants, the genes determining  the various traits were clearly dominant or recessive. However in some species, alleles determining several contrasting traits do not have a clear cut dominant-recessive relationship. This implies that neither of the alleles is completely dominant over the other.

Heterozygous individuals are phenotypically different from either of the parents. Mostly the phenotype of the heterozygous offspring is intermediate between that of the parents. This phenomenon is called Incomplete Dominance.  Examples of incomplete dominance.

1. Inheritance of flower colour in the 4 o’clock plant (Mirabilis Jalapa). If a true breeding plant producing red flowers is crossed with a true breeding plant producing white flowers, all the F1 offsprings will have pink flowers. When the F1 plants are self pollinated, they yield red flowered, pink flowered and white flowered offspring at a ratio of 1:2:1 respectively.

Diagrams

• Incomplete dominance in short horn cattle. Mating red and white shorthorn cattle yields Roan light Red) calves due to presence of both red and white hairs. A mating between two roan coloured shorthorns yields a mixture of red, roan and white coloured calves at a ratio of 1:2:1 respectively.

Diagrams

Study Question 12

Inheritance of the Blood groups (Multiple allelism)

In all the kinds of inheritance discussed so far, each phenotypic characteristic is determined by 2 variant forms of a single gene located at a specific locus on the homologous chromosome. However some characteristics are determined by more than two variant forms of a single gene. This phenomenon is referred to as multiple allelism and the genes involved are called multiple alleles. E.g. in the ABO blood groups in humans, there are three genes involved 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 for antigen B and gene O responsible for absence of antigens on the red blood cells.

Genes A and B have equal degree of dominance i.e. are co-dominant. They both express themselves when present together as in the blood group AB.

Genes A and B are dominant over gene O. Gene O is recessive and only expresses itself in the homozygous condition. The genotypes for the four blood groups in the ABO system are therefore,

Blood group (Phenotype)	Genotype	Antigens
A	AA, AO	A
B	BB,BO	B
AB	AB	A and B
O	OO	O- Zero

i.e.

• AA or AO-Blood group A
  • BB or BO – blood group B
  • AB – blood group AB
  • OO – blood group O

A marriage between a man of blood group A and a woman of blood group B will produce children of all the four blood groups if both parents are heterozygous.

Diagram

Marriage between a man of genotype AA (blood group A) and Woman of genotype BB (blood group B) results in all the offsprings having blood AB.

Diagram.

Assignment

Work out the following crosses

• Both parents with blood group O
• Heterozygous blood group A and blood group O

Study Question 13

Inheritance of the Rhesus factor

In man the possession of Rhesus antigens makes one Rh+ and this is dominant over Rh–ve. If blood from a Rhesus positive person is transfused into a rhesus negative person, this induces antibodies against the Rhesus factor of the donor. This causes agglutination of red blood cells of the recipient.

If a Rh-ve woman is married to a Rhe+ve, when she becomes pregnant, the child will be Rh+ve. Rhesus antigens cross the placenta into the mother’s blood stream. This stimulates the mother’s immune system to produce Rhesus antibodies. When these antibodies get into the foetal circulation, an antigen-antibody reaction takes place and the red blood cells of the foetus are destroyed (Haemolysed).

During the second pregnancy, the amount of Rhesus antibodies are more and cause a lot of damage to the foetus’s red blood cells resulting to death. This is called Haemolytic Disease of the Newborn or Erythroblastosis foetalis.

Determining Unknown Genotypes

This can be done in two ways.

1. Carrying out a Test Cross

A test cross is a cross between an individual of unknown genotype with an individual of a recessive genotype. A test cross where an offspring is crossed with one of its parents is called a Back Cross.

In garden pea plants the gene that determines red flowers is dominant over that which determines white flowers. A plant with red flowers may either be homozygous (RR) or heterozygous (Rr) for this characteristic. To establish its correct genotype it is crossed with a homozygous recessive plant i.e. a white flowered one (rr)

If all their offsprings bear red flowers then this indicates that the red flowered plant is homozygous or it’s from a pure line.

Diagram

If the offsprings bear a mixture of red and white flowers in the ratio of 1:1, this indicates that the red flowered plant was heterozygous.

Diagram

Selfing

Unknown genotypes can also be determined by carrying out selfing experiments. For example, a phenotypically tall plant is either homozygous (TT) or heterozygous (Tt) for this trait.

If selfed and all its offsprings are tall, the parental genotype is TT that is homozygous dominant.

Diagram

But if after selfing both tall and dwarf offsprings are produced in the ratio 3:1 respectively, then the parental genotype is heterozygous (Tt).

Diagram

Sex Determination

The sex of an organism is a genetically determined characteristic. Cells of most organisms contain a pair of chromosomes called sex chromosomes in addition to the ordinary chromosomes. In man there are 46 chromosomes (23 pairs of homologous chromosomes in everybody cell). The genes determining whether a child becomes a female or a male are located on the specific pair of sex chromosomes called the X and the Y named after their shapes.

The remaining 22 pairs of chromosomes are called Autosomes.  Autosomes are responsible for other inheritable traits.

A male human being carries the XY chromosome i.e. he is Heterogametic

The female carries the XX chromosomes i.e. Homogametic.

After meiosis in a male the spermatozoon can either carry the X or Y chromosome while the female ova contain only the X chromosome. The sex of a child is a matter of chance and depends only on whether a spermatozoon that fertilizes the ovum carries X or Y chromosome.

There is therefore a 50% chance that fertilization can result in either XY (Boy) or XX (Girl) i.e.

Female                 (XX) Male (XY)	X	X
X	XX	XX
Y	XY	XY

I.e. 2 girls: 2 Boys

In terms of probability, the chance that a boy or a girl is produced in a family is ½.

NB/ in birds the female is XY – heterogametic and the male is XX – homogametic.

In some insects, the female is XX and the male is XO with the Y chromosome absent.

In the fruit fly (Drosophila melanogaster) sex determination is as exactly as in man, i.e. male XY and Female XX.

Linkage

An organism has a large number of traits controlled by many different genes. Because the number of chromosomes is limited, each gene cannot be located on its own chromosome. Therefore chromosomes must accommodate many genes each controlling particular characteristics. Those genes located on the same chromosome are called linked Genes. All the linked genes constitute a linkage group. Linked gene are inherited together and do not segregate/separate during meiosis. They are therefore transmitted into the same gamete.

Diagram

If genes Q, R and T are linked, then all the three pairs of genes are accommodated on a homologous pair of chromosome.

In Drosophila sp, it has been found that the genes for wing length, abdomen size and body colour are located on the same chromosome. Therefore these characteristics are usually inherited together.

Sex-linked Genes

All the genes located on the sex chromosomes are said to be sex-linked. They are therefore transmitted together with those that determine the sex. In Drosophila melanogaster, the gene, which determine eye colour, is located on the X chromosome. However the corresponding allele on the Y chromosome is absent. This is because most sex-lined genes are carried on the X chromosome whereas the Y chromosome carries very few genes and is almost empty.

In humans there are few genes located on the Y chromosome, which control traits that are exclusively found in males. These are, Premature baldness and tufts of hair in the in the inner pinna and in the nose.

Diagrams.

The characteristics controlled by genes located on the X chromosome include Colour blindness and Haemophilia. These characteristics can arise in either male or females.

Colour blindness

This is the inability to distinguish Red and Green colours by some people. This trait is linked to the X chromosome. The gene that determines normal colour vision is dominant over that for colour blindness.  A marriage between a colour-blind man and a woman homozygous for normal colour vision results in their daughters being carriers but with normal colour vision. The daughters are said to be carriers because they are heterozygous and colour blindness is suppressed/masked by the dominant gene for colour vision.

All the sons are of the two parents are however normal. This is illustrated below. Let N represent the gene for normal colour vision and n represent gene for colour blindness. Since the gene is linked to X chromosome, its alleles are represented as X^N and Xⁿ.

Colour blind male                       ( XnY) Normal Woman (XNXN)	Xn	Y
XN	XNXn	XNY
XN	XNXn	XNY

All the daughters are carriers- X^NXⁿ

All the sons have normal colour vision-X^NY

If a carrier daughter from the above parents married a normal man, some of their sons will suffer from colour blindness while the daughters will either be carriers or homozygous for normal colour vision as shown below.

Carrier female                         (XNXn) Normal male (XNY)	XN	Xn
XN	XNXN	XNXn
Y	XNY	XnY

Offsprings are; X^NX^N             -Daughter with normal colour vision

                 X^NXⁿ                 -Carrier Daughter

                 X^NY                -Son with normal colour vision

                 XⁿY                -Colour blind son.

The above examples show that the gene for colour blindness is passed from mother to sons.

This is because the only X chromosome a man inherits is from the mother. If the X chromosome carries the gene for the trait, then this gene will be expressed since allele on the Y is absent. Therefore there are more male sufferers in a population compared to females.

Females only suffer when in homozygous condition of the recessive gene. Inheritance of colour blindness through several generations can be clearly illustrated using a pedigree. A pedigree is a record in table form showing the distribution of one or more traits in different generations of related individuals. Fig. 1.24

Haemophilia

This is another sex-linked trait where the blood of the sufferer takes abnormally long time to clot. There is prolonged breeding in the event of a cut hence the term Bleeder’s Disease. A recessive gene on the X chromosome causes haemophilia.

If a normal man is married to a carrier woman for haemophilia, there is a probability of ½ that if their child is a boy will be a haemophiliac and if a daughter, she will be a carrier. None of the daughters of the couple will be haemophiliacs.

Let H represent the gene for normal blood clotting and h to represent gene for haemophilia i.e.

Carrier woman                 (XHXh) Normal man (XHY)	XH	Xh
XH	XHXH	XHXh
Y	XHY	XhY

 Their offsprings will be;      -X^HX^H -Normal daughter

X^HX^h   -Carrier daughter

X^HY     -Normal son

X^hY     -Haemophiliac son.

Study question 14

Apart from carrying the sex-linked traits, the X chromosome in the females and the Y in males bring about the development of both the primary and secondary sexual characteristics. At puberty, secondary sexual characteristics in females include breast enlargement, widening of the hips, and growth of pubic hair and onset of menstrual cycle. The X chromosome controls these.

In males, they include growth of pubic hair and beard, deepening of the voice, widening of the shoulders etc.

Effects of Crossing Over on Linked Genes

Some of the linked genes separate and are transmitted on different chromosomes. This happens during crossing over (prophase I of meiosis) 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. The fusion of such gametes containing chromatids whose genes have changed places produces new combinations (recombinants). Crossing over results in chromosomal mutations, which in turn cause variations.

Diagrams

Mutations

Mutation is brought about by spontaneous changes in the individual’s genetic makeup. Mutations are normally due to recessive genes most of which are transmitted in the usual Mendelian fashion. Therefore they are quite rare. Individuals with mutations are referred to as mutants. Mutation can be induced by certain factors. Such factors are called Mutagens. They include,

• Exposure to Gamma rays
• Ultra violet light
• Colchicine
• Mustard gas

NB: Mutations occurring in gametes are more important than those in somatic cells. Mutational changes are the basis of discontinuous variation in population.

Types of Mutations

1. Chromosomal mutation
2. Gene mutation

Chromosomal Mutations

This involves the change in the structure or the number of chromosomes. During crossing over in meiosis homologous chromosomes intertwine at points called chiasmata. These points are later broken creating various opportunities for changes on the chromatids. There are five types of chromosome mutations (chromosome aberrations).

• Deletion
• Duplication
• Inversion
• Translocation
• Non-disjunction

Deletion

This occurs when some sections of chromatids break off and fail to recombine. They are therefore completely lost and the genetic material they contain is said to be deleted out. Most deletions are lethal since the offspring may lose genes responsible for the synthesis of some vital protein molecules.

Diagrams

Duplication

In this case a section of chromatids replicates and adds an extra length to itself. Duplication can produce serious effects depending on the chromosome sections involved.

Diagrams

Inversion

In this case a chromatid breaks at two points. When rejoining, the middle piece rotates and joins in an inverted position. This reverses the gene sequence along the chromatid. This might bring together genes whose combined effects are advantageous or dis-advantageous.

Diagrams

Translocation

This occurs when a section of one chromatid breaks off and becomes attached to another chromatid but of a non-homologous pair.

Translocation therefore involves the movement of genes from one non-homologous chromosome to another.

Diagrams

Non-disjunction

This leads to addition or loss of one or more whole chromosomes. If it occurs at anaphase of the first meiotic division, two homologous chromosomes fail to segregate and they move into the same gamete cell. If it happens at anaphase of the second meiotic division, sister chromatids fail to segregate. This results in half the gametes containing two of the same chromosome while the others have none.

Diagrams

Non-Disjunction causes the following

1. Downs’s Syndrome: this is where there is an extra somatic chromosome number 21. such individuals have;
2. Slit eye appearance
3. Reduced resistance to infections
4. Mentally deficient
5. Thick tongue
6. Cardiac malfunctions
7. Short body with thick fingers

NB/ these conditions are common among children born of mothers above 40 years old.

• Klinefelter’s Syndrome: in this case individuals have an extra sex chromosome. Such individuals have a total of 47 chromosomes in their cells i.e. XXY (male) and XXX (female). This occurs as a result of non-disjunction during spermatogenesis or oogenesis. The symptoms of Klinefelter’s syndrome are
• Infertility in males due to lack of sperm production
• Under developed testes
• Reduced facial hair in males
• Very tall with signs of obesity

Diagrams

• Turner’s syndrome: This is where an individual lacks one sex chromosome hence there are 45 chromosomes (XO or YO).
• Polyploidy: sometimes during meiosis chromosomes might undergo non-disjunction. This results in half the number of gametes having two of each type of chromosome i.e. diploid the rest having none. If the resulting diploid gamete fuses with a normal haploid gamete a triploid zygote is formed. If two diploid gametes fuse, a tetraploid individual is obtained. This is what is called polyploidy.

Polyploidy is rare in animals but common in plants where it’s considered to be advantageous. Polyploidy increases yields, early maturity and resistance to pests and diseases. It can be artificially induced using a chemical called colchicine, which prevents spindle formation during mitosis leading to a cell with double the number of chromosomes (4n).

Gene Mutation

This involves a change in the structure of a gene. Gene mutations are also referred to as point mutations. A gene mutation arises as a result of a change in the chemical nature of the gene. The change may involve some alterations in the DNA molecule. A change in the DNA molecule is passed onto the m-RNA. This alters the sequence of amino acids during protein synthesis. This may result in unintended protein molecules being synthesised, which may be lethal. Types of gene mutations;

• Insertion
• Substitution
• Inversion
• Deletion

Insertion

This is the addition of an extra base onto the existing DNA strand.

Diagram

By this insertion no polypeptide chain is formed as it were intended.

Deletion

This is the removal of a gene portion. If the base Thymine is deleted from its position as indicated below, the base sequence becomes altered at this point.

This results in the wrong proteins being synthesised.

Diagrams

Substitution

This is the replacement of a portion of the gene with a new portion. If Adenine is substituted by Guanine on a DNA strand, the base sequence is altered at this particular portion.

Diagrams

Inversion

If a portion of the DNA strand is rotated through 180⁰ that portion is said to be inverted as shown below. This alters the base sequence at this point.

Diagrams

Study Questions

Disorders Due to Gene Mutations

Such disorders include albinism, sickle cell anaemia, haemophilia, colour blindness and chondrodystrophic dwarfism.

1. Albinism

This a condition where the synthesis of skin pigment called melanin fails. The victim has a light skin, white hair and pink eyes. Such a person is referred to as an Albino. Melanin is derived from two amino acids – Phenylalanine and Tyrosine. Melanin is synthesised through a series of reactions controlled by a specific gene.

Gene ‘A’ is responsible for presence of melanin and ‘a’ is responsible for its absence. Gene ‘aa’ in homozygous state blocks in one or two places in the synthesis of melanin hence no melanin is formed. This occurs as a result of one enzyme (Tyronase) failing to be formed in the presence of the recessive gene.

A person with genotype AA has normal skin pigmentation.

One with genotype Aa is a carrier and has normal skin pigmentation. In a family an albino can be born under three conditions only.

• If both parents are albinos
  • If one of the parents is an albino and the other a carrier
  • If both parents are carriers

Assignment

• Work out crosses in each case.
• What is the probability of getting an albino child in each case?
• Sickle Cell Anaemia

This is a gene mutation as a result of substitution. Normal haemoglobin Hb A consists of two polypeptide chains. In the sickle cell condition, one amino acid called glutamic acid is substituted by another amino acid called valine in each of the two-polypeptide chains of the haemoglobin molecule. The resulting haemoglobin is known as Haemoglobin S – Hb S and is different from the Hb A in several ways.

Comparison between Hb A and Hb S

Normal haemoglobin (Hb A)	Defective Haemoglobin (Hb S)
1. A position in each polypeptide chain is occupied         by glutamic acid	The same position is occupied by valine in each polypeptide chain
2. Does not easily crystallise in low oxygen          concentration	Easily crystallises in low oxygen concentration
3. The haemoglobin is efficient in oxygen loading     and transportation	Not efficient in oxygen loading and transportation
4. The red blood cells are biconcave in shape	Red blood cells are sickle shaped (crescent shape)

Sickle cell anaemia is therefore the condition where the victim is homozygous for the defective gene that directs the synthesis of haemoglobin S. Most of the victims’ red blood cells are sickle shaped and the person frequently experiences oxygen shortage to the body tissues. Such a person cannot carry out strenuous physical exercises.

Many sickle cell victims die young due to insufficient oxygen supply to body tissues.

In the heterozygous condition, less than half the number of the red blood cells is sickle shaped. The rest are normal and efficient in oxygen transport. This is referred to as sickle cell trait. An individual with the sickle cell trait experiences a mild case of anaemia but leads a normal life.

Inheritance of Sickle Cell Anaemia

If a man with sickle cell trait marries a normal woman, the probability that any of the offspring will carry the sickle cell trait is ½.

If both parents are carriers the probability of getting an offspring with sickle cell anaemia is ¼.

• Haemophilia: This condition where the blood takes abnormally long time to clot. A haemophilic gene that prevents the production of the clotting factors causes the condition.
• Colour blindness: There are different forms of colour blindness. The most common one is the red-green colour blindness. In this case an individual is unable to distinguish between red and green colours.

NB. Most disadvantageous genes are recessive. Very few are dominant e.g. the gene for chondrodystrophic dwarfism

Study Question 16

Effect of Environment on Heredity

The genotype and the environment influence the development of an individual. In animals genetically identical individuals reared under different environments will appear different than those reared under very different conditions. Consider identical twins.

Practical Applications of Genetics

• Plants and animal breeding
• Blood transfusion
• Genetic counselling
• Genetic engineering
  • Plants and Animal Breeding

Man chooses those plants and animals with the desirable qualities. This is referred to as artificial selection. Inbreeding or crossbreeding does this. Inbreeding however increases the chances of undesirable genes whereas crossbreeding increases heterozygosity with the offspring’s having better performance than both parents. This is referred to as hybrid vigour e.g. a cross between Boran and Hereford.

Polyploidy has also been used in planting. The original wheat had a diploid number of 14 chromosomes but the commercial wheat has either 28 or 42 (tetraploid-4n or hexaploid-6n).

Examples of characteristics, which have been selected in agriculture.

1. Resistance to diseases e.g. cassava resistant to cassava mosaic, coffee variety resistant to CBD.
2. Early maturity in animals and plants.
3. Adaptations to various conditions e.g. rainfall, temperature etc.
4. Ease of harvesting e.g. in coffee and bananas where dwarf varieties have been developed
5. Increased productive season e.g. in chicken
6. Higher productivity
7. Production of flowers such as roses for their colour and aroma.

2) Blood Transfusion

Before blood is given to a recipient, blood typing is first done. This is done to ensure compatibility between the donor and the recipient.

Blood typing also can be used to solve disputed parentage. However the most recent technique in establishing parentage is the DNA matching.

3)     Genetic Counselling

This is the provision of information and advice on genetically inherited disorders to individuals. The individual is given such advice to enable him or her make the best choice.

Examples of disorders for which genetic counselling may be done include

• Sickle cell anaemia
• Haemophilia
• Albinism
• Erythroblastosis foetalis
• Colour blindness
• Klinefelter’s syndrome

In order to confirm the disorder the doctors can do the following

• Physical examination e.g. Lack of breasts in Turner’s syndrome.
• Laboratory tests e.g. blood tests to confirm sickle cell anaemia
• Amniocentesis for chromosomal abnormalities in foetus
• Family history may be used to determine possible inheritance of the disorder e.g. haemophilia.
• Genetic screening of the defective gene in the population

4)     Genetic Engineering

This deals with identification of a desirable gene, altering, isolating and transferring it from one living organism to another.