Genetics | Biology Form 4

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Genetics | Biology Form 4










Albinism

Albinism is a condition where the synthesis of the pigment melanin fails and is characterized by a light skin, white hair, and pink eyes. The pigment melanin is a derivative of the amino acid phenylalamine and tyrosine. The pigment is synthesized through a series of enzyme controlled reactions.

 

Each of these reactions is controlled by specific gene. In albinism the gene undergoes substitution and a recessive allele designated by a letter a. Under homozygous condition aa the enzyme tyrosinase is not synthesized and consequently melanin is not formed. Tyrosinase is necessary to act on tyrosine which is required in melanin formation.

The photograph shows a person with albinism.

The inheritance of the condition can be illustrated as follows.



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Background

We learnt in form one that Genetics is a branch in Biology dealing with the study of inheritance. We also learnt earlier that chromosomes carry the materials to be inherited. The inherited material may make an offspring have characteristics similar to the parents and at times have characteristics different from those of parents. In this topic we shall learn how inheritance takes place.

GENETICS


Introduction

Genetics is the study of heredity and variation. Heredity is the transmission of characteristics from parents to offspring during reproduction. Variations are observable differences in organisms of the same species e.g. heights in a family some may be short others tall. In this topic we shall learn about:


Continuous and discontinuous variation,


The first law of inheritance and describe Mendel's work,


Sex determination and linkage,


Mutations and


Application of genetics.

Practical applications of genetics

Knowledge in genetics has been applied in a variety of scientific fields such as:

  • Blood transfusion
  • Plant animal breeding
  • Genetic counseling
  • Genetic engineering.

Blood transfusion

This is a process of transferring blood from one person(donor) to another(recipient) in order to save the life of the patient.

 

The video clip below shows a blood donation process. Click on the play button to view the video.


Play the video clip below to view the blood transfusion process.


For the transfusion to be successful the donor's blood group should be compatible with that of the recipient. To ensure compatibility of donor and recipient blood group, blood typing is done prior to transfusion. During blood typing a qualified person tests for both ABO and RH antigens.

A compatibility chart is shown below

Plant and animal breeding

Over time, human beings have used genetic information to breed plant and animals by selectively choosing those with desired qualities. This process is called artificial selection. Through this process plants and animals artificially breed in order to: (a) Increase yields

The photographs show high yielding organisms

 

Improved resistance to pests and diseases.

The photograph shows ruiru II coffee breed which is resistant to some diseases such as coffee berry disease.

Increase rate of growth

Characteristics are selected and used to produce fast maturing and high yielding organisms.

The photograph shows fast growing and high yielding maize varieties.


Genetic counseling

This is the provision of information and advice on genetically inherited disorders their risks and outcomes. The genetic counselor presents medical and scientific facts released so that those being counseled can make their own informed decision.

Examples of disorders for which genetic counseling may be required include:

a) Sickle cell anaemia

b) Albinism

c) Hemophiliac

d) Erythroblastosis foetalis

e) Klinefelter's syndrome

f) Colorblindness.

The video shows a couple being counseled. Click on the play button to view the video.


Genetic engineering

Genetic engineering refers to a method of changing the inherited characteristics of an organisms in a predetermined by altering its genetic material. It involves identification of desirable genes altering, isolating and transferring it from one living organism to another. Genetic engineering has been applied in many areas including farming, medicine, environmental management, crime detection and sequencing of genes.

(i) Farming

Genes for production of desirable characteristic have been isolated and transferred into plants to enable them process these features such include:

Gene for synthesis of vitamin A has been transferred to rice in regions where rice is the only staple food.

The photograph shows genetically modified (GMO) rice

Genetically modified maize and soya beans have been produced which have resistance against insects and pests.; In maize production wild types of maize has been engineered to produce a high yielding and disease resistant strains.

The photograph shows genetically modified (GMO) maize

In tomatoes the gene responsible for softening of ripe tomatoes has been deleted to enable ripe tomatoes remain hard for longer making it to transported long distances and last longer.

The photograph shows genetically modified (GMO) tomatoes

(ii) Medicine


  • Human insulin has been produced from bacteria by transferring the gene for insulin production into bacteria.

  • The genes for production of blood clotting factor has been transferred form man to embryo of a sheep to interact with the gene for milk production in order for the sheep to produce milk containing factor .These factor is latter isolated and used to relieve haemophiliac

  • Human somatotrophic hormone for treatment of dwarfism has now been extracted from genetically modified strain of E. coli bacteria

Production of vaccines


  • The vaccine for hepatitis B virus has been produced. The gene for the synthesis of the viral coat of hepatitis B is isolated and transferred to a fungus. The fungi produce this protein which is latter isolated and purified before injecting into man to stimulate production of antibodies against the viru

  • Gene therapy is the replacement of faulty genes with the normal ones in order to correct the defects in man. Genetically modified organisms carry the normal gene and introduce it into the affected tissues cells. E.g. Lungs cystic fibrosis is corrected by gene therapy.

(iii) Environmental management

Decomposition of hydro carbons in petroleum is used to in controlling pollution due to oil spillage in water and soil by ship through the use of pseudomonas. The photograph shows oil spillage.

(iv) Crime detection

Particular individuals have specific DNA unique to them. Series of genetic techniques can be used to establish and produce a film on DNA patterns producing a finger print. In a scene of crime a specimen from a suspect e.g. hair, blood, semen, can be obtained and the DNA extracted and matched with suspects helping isolate the culprit DNA fingerprinting can also be used in solving disputed parentage since a child inherits one set of DNA from the mother and the other from the father. By comparing the DNA from the child the mother and the suspect father can help identify the real father and confirm paternity.

(v) Cloning.

This is a process of using an individual cell to produce a group of cells without fertilization. The offsprings are called clones. Clones are genetically identical.

The illustration shows picture of Dolly sheep

Process of cloning.

(vi) Human genome

Genome refers to the total genetic content of any cell in an organism. Knowledge on genome is important in gene mapping. Gene mapping refers to identification of specific positions occupied by specific genes on a chromosome. Gene mapping can help in isolation and correction of defects such as sickle cell gene is located on chromosome11 and haemophilia on the X chromosome.

(vii) Sequencing of genes.

The process involves analyzing the DNA to reveal the order of bases in all chromosomes which assist in:

a) Identification of defective genes

b) Identification of genes that make individuals prone to certain diseases alerting the individuals to take preventive measures.

(c) Predict the amino acids hence the proteins produced.

This facilitates production of drugs to inhibit or enhance the process.




















Concepts of genetics.

Variations within plant and animal species.

There are two types of variation Discontinuous Variation and Continuous variation.

Discontinuous variation

This is the possession of observable differences where individuals form definite distinct groups without intermediate forms e.g. an individual is either male or female, Tongue rolling, hair in the nose or ears, finger prints. Discontinuous variation is controlled by one or two major genes whose physical expression is not influenced by environment conditions.

Play the video clip to view rollers and non-rollers.


 


Continuous variation

Continuous variation on the other hand is characterized by possession of observation differences where individuals exhibit a wide range of the same characteristic from the extreme end to the other with many intermediate forms e.g. height. It is controlled by many genes whose expression is affected by the interaction with the environment.

The photograph shows individuals of the same age but different height

Causes of variations.

Variations are caused by:

  • Gene formation
  • Fertilization
  • Mutations - Sudden changes occurring in the gene or chromosomes.

Play the animation to see formation of chiasmata and crossing over in meiosis.

Chromosomes

We learnt earlier that chromosomes are thread like structures found within the nucleus which carry the genetic materials Each chromosome is made of two parallel strands called chromatids connected by a structure called centromere. All chromosomes occur in pairs looking alike and each has a characteristic length though they contain different genetic composition. The member of each pair is known as a homologous chromosome.

The illustration shows the structure of a chromosome.

Each species of animals or plants contain a definite constant number of chromosomes e.g. in man a sperm has 23 while an egg has 23 making the zygote to have 46 chromosomes.

Play the animation to view formation of a zygote.

Chromosomes exhibit certain characteristic behavior during the stages of cell division On each chromosome there are several genes which are shorter sections of DNA and each gene determines a particular characteristics of the cell in an offspring.

The genes occupy definite positions on the chromosomes called gene loci A gene is a portion of a DNA that codes for a protein. DNA is a complex molecule composed of building blocks called nucleotides.

The illustration shows a DNA molecule.

Each nucleotide is comprised of: - (i) One of the four base namely cytosine guanine Thymine and adenine. (ii) Deoxyribose sugar. (iii) A phosphate molecule.

The illustration shows the structure of a nucleotide

First law of inheritance.

Mendel Experiments

Gregor Mendel, (An Austrian Monk) carried out experiments to investigate inheritance of certain contrasting characteristics of the garden pea plants. These characteristics of the garden pea plant included i) Height of the stem-dwarf or tall ii) Texture of the seed coat-wrinkled or smooth iii) Colour of the seed coat-Yellow or green iv) Colour of the pods-green or yellow.

 


The photographs shows various characteristics of garden pea plant.

If pure breeding yellow seed pea plant is crossed with pure breeding green seed pea plant all the off springs produced yellow coloured seeds.

Play the animation to view the cross.

If the F1 yellow seed peas are crossed, the F2 generation produces are green and yellow in the ratio of 3:1. This shows that the yellow colour characteristic is dominant over the green colour.

Play the animation to view the cross.

We can conclude that in the F1 generation there is only one colour of seeds that is yellow, expressed, although one of the parents had green seeds. Such a character which is not expressed physically although it is present in the individual is called a recessive character. This can be represented in a Mendelian cross as shown on the figure.

Mendelian first law of inheritance: The characteristics of a diploid organism are controlled by alleles occurring in pairs; of such allele, only one can be carried in a single gamete.


Punnet square.

An alternative way of showing monohybrid cross is by use of a punnet square or checker board .In a punnet square the parental genotypes are written outside a graphical grid. The gametes of one parent are written along the top of the grid while the gametes of the other parent are written down the side of the grid. The products of the various fusion of gametes from the two parents are written in the appropriate boxes in the grid. Their relative ratios can be determined from the grid.

Play the animation to see monohybrid cross by use of a punnet square.

From the punnet square the phenotypic ratio can be calculated by counting the total number of seeds and determining the ratio of yellow seeds to those that are green which is 3:1. The genotypic ratio can be determined by determining the number of each genotype as a fraction of the total number of offsprings represented by possible fusions.


Play the animation to see how genotypic and phenotypic ratios are determined.

Terminologies in monohybrid inheritance

Phenotype: This is the physical characteristic of an organism resulting from the influence of the genes and environment e.g. plant can be tall or dwarf. Genotype: This is the genetic constitution of an organism. A phenotype can be produced by more than one genotype.

Examples of different genotypes are shown on the figures.

Pure breed: These are individual plants which when self fertilized produce offsprings which are phenotypically and genotypically similar. Allele: one of the gene pairs that determine a character: Allele always occurs in pairs e.g. the character for stem height tall can be produced by either allelic pair TT or Tt.

The illustration shows examples of allelic pairs.

Dominant allele: This is the allele that influences the characteristics to develop in an individual over another allele though present e.g. allele T for Tall plant will be expressed as tall plants whether in homozygous state TT or heterozygous state Tt irrespective of presence of allele for dwarfness (t) Recessive allele: This is the allele that cannot influence a characteristic to develop in an individual when present with dominant allele. It is usually represented by a small letter version of the letter representing dominant allele.

A cross between dominant and recessive genes is shown on the figure

Diploid: This is the number of chromosomes contained in somatic body cells denoted by (2n) Haploid: This is the number of chromosomes contained in the sex cells /gametes which is half the diploid number and is denoted by (n)

The illustration shows diploid and haploid cells

Homozygosity :This is where the two alleles in a genotype are similar e.g. TT or tt. Heterozygosity : This is when the two alleles in a genotype are different e.g Tt. First filial generation (F1) - These are the resulting offspring's or progeny of the initial parent in a genetic cross. Second filial generation (F2) - These are the offspring resulting from the cross of individual of the first filial generation

The first and second filial generations are shown on the following illustrations.

The second filial generation.


Complete and incomplete dominance

Characteristics in an organism are controlled by genes inherited from both parents. An allele is the alternative form of a gene located at a particular site on a chromosome. The illustration shows an example of an allele

Complete dominance

A Characteristic in an individual is controlled by the two alleles. If one of the alleles suppresses the other such that its effect are not observed in the presence of the other allele then this is complete dominance. This allele is said to be dominant while the one that cannot express itself in the presence of the dominant allele is said to recessive. An example of complete dominance is shown on the illustration.

Play the animation to see an example of complete dominance in maize plants.

A test cross is used to find out if an individual expressing a dominant allele is homozygous dominant or heterozygous in this way the genotype of an individual can be known.

To determine if unknown genotype is homozygous dominant, a tall female plant which can be genotype TT or Tt is crossed with a dwarf male plant tt. If the unknown genotype is homozygous, all F1 generation will be phenotypically tall as shown in the figure.

 

If the unknown genotype is heterozygous the F1 generation will be a mixture as shown in the figure.

A test cross where an offspring is crossed with one of the parents, to determine its genotype, is called a backcross.

Incomplete dominance

There are situations where there might be no complete dominance and each of the alleles expresses itself in an individual. An example of incomplete dominance is shown in the following cross.

Play the animation to see an example of incomplete dominance.

In this example neither of the alleles was dominant over the other; they show blending; this is incomplete dominance.


Inheritance of the ABO blood groups and rhesus.

The major blood group system in humans is the ABO blood group. Using this system Individuals can be of blood group A, B, AB or O. The blood groups are inherited. Inheritance of blood groups is controlled by three genes A, B and O. These genes occupy the same locus on the chromosome.

 

Genes A and B are equally dominant (co dominant) while gene O is recessive to both A and B. Chromosomes occur in pairs each individual can only have two of the same genes.

This is illustrated on the diagram below.

The presence of a single dominant gene results in the production of a protein called antigen on the membrane of the red blood cell. Gene A leads to production of antigen A while gene B leads to antigen B. Gene O does not result in the production of any antigen on the red blood cells.

 

The antigen on the red blood cells determines the individual's blood group. The presence of a single dominant allele also results in the blood group producing antibodies which float in the blood plasma. Gene A results in the blood producing antibody b, while gene B results in the blood producing antibody a. For example an individual with genotype AO would give rise to antigen A on the red blood cells and antibody b in the plasma.

An example is shown on the illustration.

The following are therefore the various genotypes in the ABO blood system with their corresponding antigens, blood groups and antibodies in the plasma. The illustration shows the various genotypes in the ABO blood system.

The ABO genes are inherited in a normal mendelian fashion. A man of genotype AA and a woman of BO would give the following combinations. Play the animation to see the combinations.

Half the children would be blood group AB while the other half would be blood group A.

A man of genotype AO and a woman of genotype BO would give the following combinations. Play the animation to see the combinations.

A quarter of the children would be blood group AB, a quarter will be of blood group A and a quarter will be blood group O.

The Rhesus factor.

This is a protein or antigens found on the surface membranes of red blood cells. It occur in 85% of people.15% do not have it. Those who have are described as rhesus positive, indicated by adding the positive (+) and those without are indicated by adding (-) symbol to the letter for the blood group.

The table shows various blood groups with or without rhesus factor.

The presence of the rhesus antigen on the red blood cells is genetically determined. If the male parent is AB positive and the female is OO positive, then all children of the two will have the rhesus factor (will be rhesus positive).

This cross is demonstrated on the illustration below.

If one of the parents is rhesus negative and the other parent is rhesus positive, all the children will be rhesus positive due to the rhesus gene present in the blood of the other parent. For instance a male parent AB+ and a female parent AB- will have all children being rhesus positive.

The illustration shows the cross between the two parents.

If both parents are rhesus negative all the offspring would be rhesus negative. For instance a male parent A negative and a female parent AB negative will have all children being rhesus negative.

The illustration shows the cross between the two parents.

Sex determination

As we learnt earlier, there are 46 chromosomes in every somatic cell in humans. The genes that determine whether a child conceived becomes a male or female are located on specific pair of chromosomes known as sex chromosomes. The sex chromosomes are given the symbols X and Y due to their shape.

The illustrations show X and Y chromosomes.

In females, the sex chromosomes are identical, they are of X type.

Play the animation to see the sex chromosomes in females.

In males the pair is XY which are non identical.

Play the animation to see the sex chromosomes in males.

The sex of a child is a matter of chance; it depends on whether the spermatozoon that fertilizes the ovum carries the X or the Y chromosome. The illustrations show sperms carrying different types of sex chromosomes.

During fertilization, there are four possible genotypic combinations Half of the combinations show that the child will be a girl while the other half shows that the child will be a boy.

The illustration shows sex determination in humans.


Therefore the probability that a boy or girl is produced is a half.

The illustration shows the probability of a girl or a boy being produced after fertilization

Linkage

As earlier discussed, pair of homologous chromosomes always carry genes for the same characteristic at the same place on the chromosome.

The illustration shows gene loci in a pair of chromosome

A part from determining sex of a child, the sex chromosomes have other genes as well. These genes are said to be sex linked. All linked genes constitute a linkage group. Linked genes are inherited together; they do not segregate during meiosis.

The illustration shows chromosomes with linked genes

Sex linked genes

These are genes located on the sex chromosomes and are transmitted together with those that determine sex. Most sex linked genes are carried on the X chromosome; the Y chromosome carries very few genes. In humans there are few genes located on the Y chromosome which control characteristics that are exclusively male. These include premature baldness and tuft of hair in the ear pinna and in the nose Some of the characteristics controlled by genes on the X chromosome include colour blindness and haemophilia. The two can appear either in males or females.

The photographs show some characteristics controlled by sex linked genes.

Colourblindnes.

A colour blind person is unable to distinguish between red and green colors. This condition is referred to as red green colour blindness. The trait is linked to the X chromosome. The gene that determines normal colour vision is dominant over that for colour blindness

.
This is therefore illustrated using capital letter C for normal colour vision and small letter c for colour blindness. Since the genes are linked to X chromosome, its alleles are represented as XC and Xc.

Play the animation to see the genotypes for colour blindness and normal vision.

A cross between a colour blind man and a woman homozygous for normal colour results in daughters being carriers but with normal colour vision and sons having normal colour vision The daughters are described as carriers since they are heterozygous, the X chromosome inherited from their father had colour blindness gene linked on it. They will however have a normal colour vision because the gene for normal colour vision inherited from their mother is dominant over the gene for colour blindness.

 

The illustration shows the cross between colour blind man and a woman homozygous for normal colour

Across between a carrier woman and a normal man would result in their sons being colour blind while the daughters will have normal colour vision. The illustration shows the cross between carrier woman and a normal man

Inheritance of colour blindness in many generations can be shown using a pedigree. This is illustrated in table form showing the distribution of one or more traits in different generations of related individuals.

The pedigree chart shows inheritance of colour blindness

Haemophilia.

This is a sex linked trait where the blood of the individual takes an abnormally long time to clot after an injury. The result of this is usually prolonged bleeding hence the term bleeders' disease. Haemophilia is caused by a recessive gene on the X chromosome. This condition is normally illustrated using capital letter H for normal blood clotting condition and small letter h for haemophilia. Since the genes are linked to X chromosome, its alleles are represented as XH and Xh.

Play the animation to view the genotype and phenotype of a haemophilic and a person with normal blood clotting.

In a cross between a man with normal blood and a woman who is a carrier for haemophilia gene but with normal blood clotting, some of the daughters will have normal blood clotting and others will be carriers .For the sons some will have normal blood while others will be haemophiliac.

The illustration shows a cross between normal man and a carrier woman.


The following table shows the genotypes and phenotypes of the possible individuals

Mutation

It refers to spontaneous change in the individual genetic make up. It may lead to individuals with some unusual characteristics which may be beneficial, neutral or harmful. Mutations are normally due to recessive genes. Mutations can occur naturally. They can also be induced by certain factors called mutagens. Examples of mutagens include: radiations, mustard gas, colchine, hydrocarbons.

Types of mutations.

There are two types of mutations

i) Chromosomal mutations

ii) Gene mutations.

Chromosomal mutations

Chromosomal mutation may be the result of change in the number or structure of chromosomes. The five types of chromosomal mutation are:

a) Deletion

b) Duplication

c) Inversion

d) Translocation

e) Non disjunction

Deletion

Occurs when some section of chromatid breaks off and fails to reconnect to any of the chromatids. Deletion involves loss of genes.

Play the animation to see how deletion occurs

Duplication

In duplication, a section of a chromatid replicates and adds an extra length to itself. This leads to repetition of set of genes.

Play the animation to see how duplication occurs

Inversion

Inversion occurs when a chromatid breaks at two points then rotates through 180 degrees then rejoins in an inverted manner. The nucleotide in the sequence is reversed.

Play the animation to see how Inversion occurs

Translocation

Occurs when a section of a chromatid breaks off and becomes attached to another chromatid but of the non homologous pair. It involves movement of genes from one non homologous chromatid to another.

Play the animation to see how translocation occurs

Non disjunction

Non disjunction leads to addition or loss of one or more whole chromosomes. During anaphase I the two homologous chromosomes may fail to segregate and so move on to the same gamete cell. If the sister chromatid fail to segregate at anaphase II, the two chromatids move to the same gamete.

Play the animation to see how non disjunction occurs

Disorders due to non disjunction.

Non disjunction may lead to various disorders referred to as syndromes. Such syndromes include down's syndrome, kleinfelter's syndrome and turner's syndrome.

The photograph shows examples of disorders that result from non-disjunction.

Down's syndrome

In Downs syndrome, the individual has extra somatic chromosome number 21. The symptoms of Down's syndrome are: Slit eye appearance, thick tongue, Short body with stubby fingers, reduced resistance to infection, mentally deficient, cardiac malfunctions.

The photograph shows down's syndrome.

Kleinfelters syndrome

Individuals with Kleinfelters syndrome have an extra sex chromosome hence they have a total of 47 chromosomes in their cells. Symptoms of Kleinfelters syndrome in males include Underdeveloped testes Breast development reduced facial hair taller than average with signs of obesity. Persons with XXX constitution appear relatively normal in most characteristics. Symptoms of Kleinfelters syndrome in females include Underdeveloped ovaries, Characteristic facial features, Constriction of the aorta, Web of skin on the neck and Poor breast development.

The photograph shows a person with Kleinfelters syndrome.


Turners syndrome

Results when individuals have one of the sex chromosomes missing due to non- dysfunction the individual therefore has 45 chromosomes instead of 46 YO zygotes do not develop due to absence of many vital genes.XO are females with underdeveloped female characteristics some of which include Rudimentary ovaries and testes Small uterus Broad chest and widely spaced nipples Low hairline Low set ears Increased weight-obesity Small finger nails No breasts development Short in stature. Attention deficit disorder( problems with concentration and attention)


The photograph shows a person with Turners syndrome.

Gene mutations

Gene mutation involves change in the structure of a gene. It arises as a result of a change in the chemical nature of the gene. The change may also involve alteration in the DNA molecule. This alters the sequence of Amino acids during protein synthesis. The result is unintended protein molecule which may bring confusion or may be lethal. Gene mutation includes: -

    Insertion
    Deletion
    Substitution
    Inversion

(a) Insertion

It refers to addition of a base into an existing DNA strand. This alters the amino acid alignment. Play the the animation to see how Insertion occurs.

(b) Deletion

It refers to removal of a gene portion. For example, if a gene is deleted from its position, the base sequence becomes altered.

Play the the animation to see how Deletion occurs

c) Substitution

It refers to replacement of a portion of the gene with a new portion. E.g. if A is replaced with G on a DNA strand, the base sequence is altered.

Play the animation to see how Substitution occurs.

d) Inversion

A portion of DNA rotates 180 degrees. The inverted portion rotates resulting in alteration of base sequence.

Play the animation to see how Inversion occurs.

Disorders due to gene mutations

In the earlier lesson you learnt that there are many types of gene mutations. This mutations lead to disorders in the human body which are hereditary. The common hereditary disorders include:

    Albinism
    Sickle-cell anaemia
    Haemophilia
    Colour blindness

Sickle cell anaemia

This is a condition where the red blood cells contain abnormal hemoglobin making the red blood cell to have a sickle shape instead of the normal disc shape.

The illustrations show sickle shaped red blood cells and Normal red blood cells.

The Photograph shows a sickle shaped red blood cells

Normal haemoglobin consists of two polypeptide chains containing glutamic acid. When glutamic acid is substituted with valine a recessive hemoglobin results. It is known as hemoglobin type s (Hbs). A person who receives the defective gene from both mother and father develops the disease sickle cell anaemia. An individual who receives one defective and one healthy allele remains healthy but a carrier.

 

The inheritance of sickle cell anaemia is shown in the illustrations. Let S represent the dominant allele for the normal hemoglobin in red blood cells and s represent the recessive allele for abnormal hemoglobin in sickle shaped blood cells.

Haemophilia

This is a sex linked trait where the blood of the individual takes an abnormally long time to clot after an injury. The result of this is usually prolonged bleeding hence the term bleeders' disease. Haemophilia is caused by a recessive gene on the X chromosome. This condition is normally illustrated using capital letter H for normal blood clotting condition and small letter h for haemophilia. Since the genes are linked to X chromosome, its alleles are represented as XH and Xh.

Simple mammalian reflex action.

When a person touches a flame the heat of the flame is the stimulus. It stimulates the nerve endings in the skin .These are the receptors. Nerve impulses are generated by the skin receptor neurones to the spinal cord. In the spinal cord ,the impulses are transmitted to the neurone to the motor neurone, intermediate. Impulses leave the spinal cord along the motor neurone to the effector the biceps muscles of the arm. The biceps muscle contracts to cause sudden withdrawal of the hand from the flame.

The video clip shows a person withdrawing the hand from a candle flame. Click on the play button to watch the video.


Play the animation to view a animation of a reflex arc.



Objectives

By the end of this topic you should be able to:

  1. Distinguish between continuous and discontinuous variations
  2. Describe the structure and properties of chromosomes
  3. State the first law of inheritance and describe Mendel's work.
  4. Construct and use punnet square
  5. Distinguish between F1 and F2 generations, genotype and phenotype, haploidy, homozygosity and heterozygosity, dominance and recessiveness, linkage and sec linkage, mutations and mutagens.
  6. Predict and explain the inheritance of the ABO blood groups and rhesus (Rh) factor.
  7. State examples of genetically inherited disorders
  8. Explain causes of chromosomal mutations
  9. Explain the practical application of genetics

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