GENETICS FORM FOUR

TOPIC :GENETICS
SUBJECT :BIOLOGY FORM FOUR



Genetics Is a branch of biology which deals with the study of heredity and variations in organisms.

Heredity refers to the transmission of genetic information from one generation to another. That is, the passing on of features or characteristics from parents to offspring or progeny. In humans, for example, features such as hair colour and body shape of the parents can be passed on (inherited) to their children. In genetics, a feature that can be inherited by the offspring from the parent is known as a trait. Thus, features such as hair texture, hair colour, and skin colour are all traits.

The hereditary characteristics are passed on from parents to their offspring through distinct units called genes. Genes are hereditary materials or factors, which determine a specific characteristic or trait in an organism.

Variations are the observable differences in organisms of the same species. Living things arise from other living things of the same species through reproduction. However, organisms show a great number of variations. No two organisms are exactly the same. The variations may be due to mutations of genetic material (DNA) caused by x-rays, gamma rays, ultra radiations or radioactive elements. Variations may also occur during gamete formation and combination of gametes at fertilization.

Genetics, therefore, attempts to explain either how organisms do resemble their parents in certain features or differ from their parents in other features.

Common terms used in Genetics

In genetics, there are several terms that are often used to describe different genetic features, variations or phenomena. Defined below are some of the common terms used in genetics.

1. Genotype: Is the genetic constitution or make up of an organism.
2. Phenotype: Is the outward or physical appearance of an organism.
3. Dominant gene: Is a gene that prevents the expression of another gene.
4. Recessive gene: Is a gene that is masked by another gene.
5. Homozygous: Is a condition where by the two genes for a given trait are similar/alike
6. Heterogeneous: Is a condition where the two genes for a trait are different.
7. Gene: Is a part of chromosome that carries the genetic material called DNA. Are also referred to as nucleotide chemical units of inheritance arranged along the chromosomes. They are called hereditary factors.
8. Trait: Are characteristics inherited by individual from their parents
9. Allele: Is an alternative form of a gene controlling the same characteristics but produce different effect. Example: T-tallness and t- shortness
10. Monohybrid Cross: Are offspring produced by crossing two individual with different character e.g. homozygous green-podded plant (GG) and homozygous yellow-podded plant (gg)

11.  FIRST FILLIAL GENERATION (F1): Is the first generation of offsprings produced after crossing the parental genotypes.

12.  SECOND FILLIAL GENERATION (F2): Are offspring produced by selfing the F1 generation

13.  MONOHYBRID INHERITANCE: This is inheritance of one pair of contrasting (different characteristics e.g height where an individual is either tall or short).

14.  DIHYHIBRID INHERITANCE: This is inheritance of two pairs of characteristics. Example: Pure tall pea plant with colours flowers and dwarf pea plant possessing white flowers.

15.  EPISTASIS: It is the interaction between the two different known as allelic dominant genes

16.  PEDIGREE: Is the historical or ancestral record of individuals shown in a chart table or diagram

17.  CHROMOSOMES: They are thread like structures found in the nucleus of the cell they are only visible when a cell nucleus is about to divide. Every nucleus of the cell of the same species has a constant number of chromosomes e.g. Drosophila has 8 chromosomes, fruit fly pea plant has 40chromosomes sheep has 56 wheat has 14 chromosomes maize has 20 chromosomes.

 Each member of the chromosome pair is known as homologous chromosome

Types of chromosomes

There are two types of chromosomes in the human body

1. Autosomes

These are also known as autosomal chromosomes. They carry all genetic information except that of sex. In humans autosomes are 44 in numbers forming 22 pairs.

2. Heterosomes

These are also known as sex chromosomes these chromosomes determine the sex of the organism in humans. One pair is responsible for the determination of sex

Diploid and haploid nuclei

Diploid nucleus has the chromosomes occurring as homologous pair e.g 23 pairs in the human this is denas 2n diploid nuclei are found in the gametes.

Haploid nuclei have only one set of unpaired chromosomes. In 23 chromosomes are there haploid nuclei are denoted as n diploid cells are formed after fertilization.

GENETIC MATERIALS

Genes are nucleotide chemical units of inheritance arranged along the chromosome and are capable of being replicated and mutated.

Each gene occupies a specific location on a chromosome this location is known as locus (plural is loci) each chromosome contains many genes. Homologous chromosomes when paired together will have similar or different genes called alleles.

Alleles is an alternative form of gene controlling the same character out producing different effects. The gene can control color of the skin.

NUCLEIC ACID

Nucleic acids are polymeric macromolecules or large biological molecules, essential for all known forms of life. Nucleic acids, which include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are made from monomers known as nucleotides. Each nucleotide has three components: a 5-carbon sugar. a phosphate group, and a nitrogenous base. If the sugar is deoxyribose, the polymer is DNA. If the sugar is ribose, the polymer is RNA.

Together with proteins, nucleic acids are the most important biological macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information in other words, information is conveyed through the nucleic acid sequence, or the order of nucleotides within a DNA or RNA molecule. Strings of nucleotides strung together in a specific sequence are the mechanism for storing and transmitting hereditary, or genetic, information via protein synthesis.

DNA (deoxyribo nucleic acid)

DNA has a double stranded shape or coil twisted like a ladder to form a double helix. DNA is the genetic material contained in the genes. DNA is called the “molecule of life”. This is because it determines the physical and behavioural characteristics of an organism. The DNA determines example the colour of your hair, eyes, skin, ears and nose, height, ability or inability to roll the tongue all.

Structure of DNA

DNA is a double stranded helical (spiral) molecular chain of a nucleic found within the nucleus of a cell. By “double stranded helical” it means that the DNA consists of two strands, which twist around each other in a spiral fashion.

The DNA is made up of many nucleotides forming a polynucleotide chain. Polynucleotide means many nucleotides. The polynucleotide chain runs in the opposite direction. Each chain is joined to the other by pairs of bases. There are four bases namely Guanine (G), Cytosine ©, Adenine (A) and Thymine (T)

Components of DNA

• Deoxyribose sugar
• Phosphate group
• Organic base or Nitrogenous bases.

Nitrogenouse base

• Adenine (A)
• Guanine (G)
• Uracil (U)
• Cytosine (C)
• Thymine (T)

Functions of DNA

1. There are genetic materials which are responsible for genetic characteristics
2. They assemble the amino acids to form a protein molecule

RNA (ribonucleic acid)

The RNA molecule is responsible for carrying genetic information from the DNA molecule to the ribosome which is the sight of the protein synthesis. The chromosomes determine the type of protein synthesized. The genes determine the actual characteristics of the organisms. In protein synthesis deoxyribonucleic acid acts as a template for the formation of ribonucleic acid (RNA).

Structure of RNA

RNA consists of only a single strand of polynucleotide. The polynucleotide is made up of many nucleotides. Each nucleotide consists of a nucleobase, ribose sugar and phosphate group. The RNA sugar is ribose and not deoxyribose. Its nucleotides contain only one of four bases that are:

1. Guanine (G)
2. Cytosine (C)
3. Adenine (A)
4. Uracyl (U)

NB: Uracyl replaces the thymine of DNA. So in this the adenine can pair with uracil while the Guanine pairs with thymine.

Types of RNA

1. Messenger RNA — carries information from the nucleus in from of base triplets.
2. Transfer RNA — It transfers the appropriate amino acids to the ribosome.

DIFFERENCE BETWEEN DNA & RNA

DNA	RNA
Has a deoxyribose sugar	Has a ribose sugar
Has a double stand	Has a single stand
Found in the nucleus, mitochondria and chloroplast	Found in nucleus and cytoplasm
Has organic bases, cytosine, guanine, adenine and thymine.	Has organic bases, cytosine, guanine, adenine and uracil.

PRINCIPLES OF INHERITANCE (Concept of inheritance).

Historical background of genetics

Gregor John Mendel advanced the principles of inheritance. In 1856 – 1863 Mendel grew and tested some 29,000-pea plants. From these studies, he formulated the law of segregation and the law of assortment.

After his work on peas, Mendel began to experiment with honeybees. However, he failed to produce a clear picture of their heredity because of difficulties in controlling the mating behaviour of queen bees.Mendes works was largely criticized and generally rejected during his lifetime. It was only after his death that his work gained broad recognition. He is now considered the father of modern genetics, Mendel diedMendel died on January 6th, 1884.

Mendel’s experiment

Mendel has selected garden pea plants [pisum sativa]

Reasons for selecting pisum sativa

1. The garden pea has many contrasting and easily recognized characteristics.
2. The hybrid obtained from the cross fertilization was fertile
3. The flowers of a garden pea are bi sexual and naturally self pollinated
4. The garden pea plant matures relatively fast producing many offsprings (seeds)

MENDELIAN INHERITANCE.

1.      LAW OF SEGREGATION

It states that “characteristics of an organism are controlled by internal factors (genes) occurring in a pair is earned in each gamete”

There are four main concepts in this law:

1. Genes can exist in more than one form
2. An organism inherits two alternative form of a gene for a particular trait, one from each parent
3. When the two alleles in a pair are different one is dominant while the other is recessive. This condition is called complete dominance.

When inheritance of one pair of characteristics is studied at a time it is called Monohybrid inheritance.

2.      LAW OF INDEPENDENT ASSORTMENT

“Each of the 2 alleles of one gene may combine randomly with either of the alleles of another gene independently”

PUNNET SQUARE

Is a chart showing the possible combination of factors among the offspring of a cross. It is used to show the formation of zygotes.

Female gametes are placed on the right while male gametes are placed on the left side.

Example:

A cross between homozygous tall (TT) and homozygous dwarf (tt) plant can be illustrated as follows:

Let assume tall is male and dwarf is female

Test cross

A cross-used to cross an individual of unknown genotype with a homozygous recessive individual

Example: A homozygous dominant individual (TT) will phonotypical appear the same.

BACK CROSS

In the crossing of individual of unknown genotype with the homozygous parent. This is another form of test cross, but the difference is that in test cross, it is crossed with any individual while in back cross with a parent .g: if the individual is homozygous (bb)

DOMINANCE

Dominance is a state of one character gene from one parent making the corresponding character from another parent.

Types of dominance

1. Mendelian inheritance - Complete dominance
2. Non-Mendelian inheritance - Incomplete dominance
3. Co-dominance

1. COMPLETE DOMINANCE

• Is the dominance where by one gene masks the expression of the other gene.
• A dominant gene always masks a recessive gene when the two occur together.

EXAMPLE:

1. A man homozygote for brown iris marries women who has blue iris. Show the results of F1. What colour would the iris of the cross between 2 members of F1?

Solution:

The gene for brown iris is completely dominant over gene for blue iris in woman.

Let gene for brown be B and b for blue

Genotypes - All are Bb.

Phenotype - All have brown iris.

Selfing F1

Genotypes - BB, Bb,bb

Phenotypes - 3 Brown iris, 1 brown iris

Genotypic ration - 1: 2: 1

Phenotypic ratio - 3:1

    BBBbbb

2. A pure purple flowered pea plant was crossed with pure white pea plant. Offsprings for Fl were phenotypically all purple flowered plants when F1 was selved a mixture of purple pea flowered and white pea plant were produced at an approximate ratio of 3:1

Solution:

Let gene for purple be P and white be p

Genotypes: all are Pp

Phenotypes : all have purple flower

Self  F1

Genotype – PP, Pp, pp

Phenotypic ratio - 3:1

Genotypic ratio - 1:2:1

2. INCOMPLETE DOMINANCE

In incomplete dominance there is no dominant or recessive gene, but both express themselves equally. It results in a heterozygous individual which does not resemble any of the heterozygous individual which does not resemble any.

Example:

1. A red flowered rose was crossed with white rose and all members of F1 were pink. When pink were selfed, a mixture of red, pink and white flowered plants were obtained.

Solution:

Let, R- Red, G - White

Genotypes: all are RG

Phenotype: all are pink

Genotypes – RR, RG, GG

Genotypic ratio - RR: RG: GG

                             1:2:1

Phenotypic ratio - 1 red: 2 pink: l green

3. CO-DOMINANCE

In co- dominance genes from both parents are dominant and are phenotypically expressed in the offspring.

Example: A red cow is mated with white bull. In F1 generation all of offspring have equal patches of red and white fur. Therefore neither red nor white gene is dominant over the other such cattle and called Roan.

When a roan cow is mated with roan bull, offsprings may be red, roan or white mated in the ratio of 1: 2: 1

Let: W - white bull

       R - Red cow

Genotypes - all are RW

Phenotype - all are Roan

Phenotypes ratio - 1 Red: 2 Roan: 1 White

Genotypic ratio- RR: RW: WW

                             1 : 2:1

SIMPLE MENDELIAN TRAITS

The following are example of Mendelion’s traits in man

1. ALBINISM

Albinism is absence of pigmentation melanin in human skin animals or plants. This pigmentation is responsible for dark colour of the skin. As a result the person has white hair, pink eyes and light skin. In plant are characterized by lack of chlorophyll.

It is controlled by a recessive gene. Human showing this disorder must be homozygous recessive. Heterozygous are normal but career.

Albinism in Plants

Albinism in Tiger

Examples

1. What will be the result of normal man who married an albino woman?

Solution:

Let gene for normal be A and Albino be a

Phenotype - all are normal (Heterozygous)

2. What would be the result of a cross between heterozygous parents?

Genotype - AA, Aa and aa

Phenotype - normal man, carrier and albino

3. What would the result be of crossed between heterozygous parent with an albino parent.

Solution:

Gene : Aa - heterozygous parent

            aa - albino parent

Genotypes - Aa and aa

Phenotypes - half normal/carries and albinos.

4. What would be the result of crossed between heterozygous parent and homozygous normal parent

Solution:

Heterozygous - Aa, homozygous AA

Genotypes - AA, Aa

Phenotypes - all are normal (normal, carriers)

2. ACHONDROPLASIA

Achondroplasia is a disorder that is characterized by a shorted body, legs and hands. It is controlled by a dominant gene. Individuals with these disorders are homozygous dominant or Heterozygous. Homozygous recessive are perfectly normal

Examples:

1. What would be the result of a normal man who married an achondroplasia woman.

Solution

Genes for normal man - aa

Genes for achondroplasia women - AA

Phenotypes - All are anchondroplasia

Genotypes - Aa

2. What would be the result of a cross between an achondroplasia woman who is homozygous and achondroplasia man who is heterozygous?

 Solution:

                     - AA, Aa

Phenotypes - All achondroplasia

3. What would be the result of a cross between heterozygous parents?

Solution:

Phenotypes - 3 Achondroplasia, 1 Normal

Genotypes – AA, Aa, aa  

Phenotypic ratio - 3:1

Genotypic ratio – 1:2:1

3. HAEMOPIHLIA

Haemophilia is a hereditary trait characterized by delayed blood clotting. The result is prolonged bleeding even small injuries can lead to death. The haemophilic girl rarely lives beyond puberty because of excessive menstrual bleeding. It causes high mortality rate.

It is controlled by recessive gene. Heterozygous are normal carries but homozygous individuals are haemophilic.

Worked example:

If a normal man married a haemophilic woman, the offspring’s would be

Solution:

Let genotype for the man xH Y and woman XhYh

• A haemophilic man will be Xhy
• Haemopholic female will be XhXh
• H- not suffering from haemopholic while h- haemopholic

4. COLOUR BLINDNESS

Is the hereditary trait characterized by inability to detect certain colours of the spectrum. The common colour blindness is inability to distinguish between red from green. It is controlled by a recessive gene. Homozygous individual are colour blind while heterozygous are normal or carrier.

e.g . If a colour blindness man marries a normal woman, the offspring will be as follows.

Let B - normal

     b - Colour blind

5. SICKLE CELL DISEASE

This is a genetic disorder which makes the red blood cell acquire sickle shape under certain conditions. It may occur when the person is attacked by certain diseases. e.g malaria. Also when oxygen tension in the atmosphere is very low. The sickle cells ability to carry oxygen is reduced. It is controlled by a Recessive gene. Homozygous individuals are sickle cell while heterozygous individuals are normal carriers.

NOTE:

HbA- perfect normal

HbS -  sickle cell trait

* If a carrier man marries a carrier woman the offspring will be - sickle cell anaemia.

6. TONGUE ROLLING

This is a hereditary trail which is characterized by rolling a tongue into a U - shape. It is controlled by a dominant gene. Heterozygous and homozygous individuals are tongue rollers. Recessive are not tongue rollers.

TRAITS/DISORDERS AND THEIR CONTROLLED GENE

	DOMINANT GENE	RECESSIVE GENE
	Achondroplasia	Haemosphilia
	Tongue rolling	Colour blind
	Night blindness	Sickle cell
	Brown iris	Blue iris
	Having more than 5 fingers and toes	Normal night vision
		Albinism
		Normal number of fingers

HOW TO SOLVE GENETIC PROBLEMS BY USING PUNNET SQUARE

1. In human beings normal skin pigment (melanin) is dominant over albinism. An albino male mates with a heterozygous female. If the female gives birth to 6 fraternal twins what will be the probable genotypic and phenotypic ratio of the offspring?

Solution:  

i)                    Let letter  A- dominant gene

                a- recessive gene (albinism)

Write the genotypes of the parents

                   (male) aa x Aa (female)

ii)                  Use this genotype to complete the punnet square

iii)                Summarize the genotypic and phenotypic ratios

Genotypic ration -  Aa: aa = lAa: laa

Phenotypic ration - 1/2 normal skin pigmented: albino = 1:1

2. In human beings normal skin pigment is dominant (A) over albinism (a) one couple with normal pigment mate and produce six fraternal twins. Out of 6, 4 have normal skin pigment and 2 are albino. What are the genotypes of the parents?

Solution:

1.      Write complete/partial parents genotypes and offspring

Parents A

Four normal skin offspring A

Since normal skin is dominant, each of parent and 4 children must have at least one dominant gene

2.      Since albino gene is recessive, 2 albino offspring are homozygous recessive (aa)

• Two albino offspring (aa)
• A - (Normal skin parent) x             A - (normal skin parent)
• A - (4 normal offspring aa              - (2 albino offspring)

Since one gene for albino comes from each parent. Therefore each parent is heterozygous (Aa)

RHESUS FACTOR

About 85% of the human population has a gene located on the chromosomes number one that produces a function protein called ANTIGEN & (Rhesus factor). Individuals with rhesus factor are rhesus positive (Rh+) and the remain 15% do not have this factor are rhesus negative (Rh­). Rh+ is dominant over Rh-.

Rhesus antibody is normally absent in plasma of human blood. The Rh- people produce this antibody if Rh+ blood is transfused to them. These Rh+ antigens react with rhesus antibody causing agglutination. The present or absent of Rh factor gives the blood groups the + or ­ signs.

The table below shows the reactions of blood types with and without Rh factor.

KEY: (√ ) - No agglutination

          (x) - Agglutination

WORKED EXAMPLE

A Rh+ man marries a woman who is Rh- and produces 10 children, what will be the phenotypes of the children

SEX INHERITANCE

Sex is a phenotypic character; it is dependent upon the genotype and environment. In sexually reproducing organisms, each individual is a product of a male and a female. Each individual receives an equal number of chromosomes from male and female body. For example each individual receives 23 chromosomes from the mother and 23 from the father.

• In many species female chromosomes (sex) are XX and male are XY

The chromosomal mechanism of sex determination varies in different organisms

Example:

SEX DETERMINATION AND INHERITANCE

Sex of a child (man) is determined by sex chromosomes. Human being has 46 chromosomes (23 pairs of homologous chromosomes) in every body of these. 2 are sex chromosomes while 44 are referred to as autosomes. Autosomes determine physical characteristics such as height and body size. There are two types of sex chromosomes which are X and Y. These chromosomes determine the sex of a child.

• The male carries X and Y-chromosomes which are different in shape and size and are said to be Heterogametic. The male genotype is XY.
• The female carries two X -chromosomes which are similar in shape and size and are said to be Homogametic.
• A sperm (male gamete) has either an X or Y-chromosomes while the ovum (female gamete) always contains the X chromosomes.
• Secondary sexual characteristics of females are controlled by genes on the X chromosomes.
• Male secondary sexual characteristics are controlled by genes on the Y chromosomes.
• The sex of a child is a matter of chance and depends on whether the sperm that fertilizes the ovum carries a Y or a X chromosomes. The chances of a baby being a girl or a boy are
• 50:50.
• Maleness depends upon the presence of Y-chromosomes and Femaleness depends upon the absence of the Y-chromosomes.

Sex determination in human.

The ratio of boys to girls is 1:1. This means that the probability of getting a boy or a girl is 50%.

SEX - LIMITED CHARACTERS

These are characters that are restricted to only one sex, either males or females.

Examples of sex-limited characters:

• Growth of facial hairs (Beard and Moustache) in males. This develops as a result of production of male hormones. The gene for beards growth is also present in females but it is not expressed.
• Baldness in males.
• Breast development in females (lactation).
• Long hairs of male lions (Male: lion, Female: lioness)
• Comb plumage of hens (Male: cork, Female: hen)
• Hairy ears and nose is a common characteristics among males especially those of Asiatic descent. The fact that the characteristics are only present in the males, suggests that the gene responsible for the trait is located on the Y-chromosomes.

SEX - INFLUENCED CHARACTERS.

Are the characters that are expressed as dominant in one sex and recessive in the other. These are characters or traits that tend to be more conspicuous in one sex than the other. An example of sex - influenced characters is the presence or absence of horns in some breeds of sheep.

• The horned condition behaves as dominant in males but as recessive in females.
• The hornless state is dominant in the female sex but recessive in the male.

Note: The dominance difference of sex-influenced characters is mainly the result of hormonal interaction with the genotype.  

SEX PREFERENCE AND SEX SELECTION.

Sex preference is favoring one sex (gender) and not the other.

Sex selection means choosing the sex (gender of the baby to have.

Therefore, sex preference and selection result into people to like one type of sex more than other. This tendency is very common in African countries and some parts of Asia. Basically, both males and females are equal and depend on each other in many aspects of life. However, there has been a tendency of some people to prefer one type of sex over the other. Some people in families prefer having boys than girls while others prefer girls over boys.

• Those who prefer boys do so in a belief that boys will perpetuate the lineage and take care of the parents when females are living far away with their husbands.
• Those who prefer girls argue that, girls are kind and mercy, therefore they can take care of their parents at old age.

Socio - cultural factors that influence sex- preference and sex selection

1. Manpower generation. Some societies, especially pastoralists prefer boys over girls because boys help in animal grazing.
2. Generation and protection of wealth. In some societies girls are more preferred than boys because they generate wealth upon getting married. A family will get a lot of cattle or money as a bride price.
3. Land ownership. In some societies a woman cannot own land, thus females prefer to have more sons than girls so that they can somehow benefit indirectly through their Sons.

Conclusion;

Sex preference and selection have negative impact as it may result into in equality and discrimination. In many societies, sex preference and selection has led to boys being educated and given ample time to play and learn while girls stay at home and do house chores. Government and NGO’S have to take measures to rectify the situation.

SEX LINKAGE

Sex linked genes carried on sex chromosomes but have nothing to do with sex. Traits whose expression is governed by sex-linked traits are called sex-linked traits.

One kind of colour blindness is an example of sex linked trait in human beings located on the X — chromosome. Example of other linked is haemophilia (bleeder’s diseases).

VARIATION

The difference that exists between living organisms is called Variation. It is the possession of characteristics which are different from the parent and other offspring.

Types of variation.

1. Continuous variation

Is the variation which show intermediate form between any two extremes i.e there is no clear cut distinction between two extremes. Example in group length ranges from shortest to tallest with several intermediaries continuous variation arises from interaction between genes and environment.

2. Discontinuous variation

Is the variation which show clear cut distinction from one form to another form. Example: In human population an individual is either a male or a female, ability to roll the ton2ue, albinism, blood group (A, AB, O) and rhesus factor.

Environment does not influence the characteristics that show discontinuous variation. Example blood group cannot be altered by environment.

Cause of variation

1. Environment Factors

Food — lack of food of a certain diet leads to deficiency diseases such as Kwashiorkor. Lack of enough food causes starvation. Also pathogens cause diseases in organism making the individual different from the normal ones.

2. Genetic factors

(a) Meiosis — during meiosis there is segregation of different gametes.

• This reduces the chance of pairs of chromosomes producing a wide variety of different gametes. This reduces the chance of individuals being the same.

(b) Fertilization — during fertilization the nuclei of male and female gametes fuse.

• This permits parental genes to be brought together in different combinations.
• This may lead to desirable and undesirable qualities of parents be combined in the offspring.

(c) Mutation - This is a sudden change in gene which can be inherited are caused by mutagens as x-rays, cosmic rays, chemicals as mustard gas. The individual is called a mutant after undergoing mutation and appears different from the rest of the population.

3. Migration

As species are not normally informally distributed but occurs in small isolated population called demes. If members from the deme migrate and mate with members of another deme the offspring that results have characteristics that are different from those of both parents.

TYPES OF CHARACTERS

1. Acquired characters

These are traits an individual develops as a result of adaptation to the environment. Example: Walking style. They are never inherited and are also know as no-heritable characteristics

2. Inherited characters

Are traits passed on from parents to the offspring through sexual reproduction. Are also called heritable characteristics.

Difference between acquired and heritable

ACQUIRED CHARACTERISTICS	HEREDITARY CHARACTERISTICS
1.      Are due to the environment 2.      Cannot reappear in offspring. 3.      Sometimes are changeable in life time (one way lose weight)	1.      Are due to genes 2.      Re-appear in offsprings 3.      Mainly unchangeable in life time (height)

GENETIC DISORDERS.

MUTATION

Mutation are changes in the genetic material in the gametes. It includes appearance of new characters that has never been before in that population. Individuals who undergone mutation are called Mutants.

Mutation can be due to

1. Change in a gene itself
2. Change in arrangement of genes
3. Loss of chromosomes (due to unbalanced meiosis)

Mutation can be caused by agents known as Mutagens.

• X-rays
• Cosmic rays
• Heavy metal (lead & mercury)

TYPES OF MUTATION

1. GENE MUTATION

Gene mutation occur ass a result of altering the chemical structure of genes. There is a change in the sequence of nucleotides is the segments of DNA corresponding to one gene. This in turn alters the sequence of amino acids required in synthesis of a particular protein.

The protein formed will be different from the normal ones and produce profound effects on both the structure and development of an organism Example: sickle cell, dwarfism.

TYPES OF GENE MUTATION

• Substitution
• Insertion
• Deletion
• Inversion

i. SUBSTITUTION

This is the replacement of one or more portions of a gene with a new one. E.g. A thymine (T) on ATA on the DNA molecule is replaced by cytocise (C) and result to ACA on the DNA.

This is exemplified in sickle cell anemia only one nucleotide is changed. This kind of mutation involving the change .f one nucleotide is called Point Mutation.

ii. INSERTION

This involves adding a new portion of a gene to an existing one. Example: If the base Guanine (G) is inserted between two Adenine result into AGA which does not code for any amino acid.

iii. DELETION

Deletion is the remove of a portion of a gene Example: -If base Guanine (G) is deleted in a base triplet CGC resulting into alteration of base sequence reducing the number of amino acids.

iv. INVERSION

A portion of DNA strand cuts and rotates through 1800 the inversion results in alteration of the base sequence at this part. Example: A base triplet CTA can have its base thymine (T) and Adenine (A) cut and rotated. The result is CAT which is different from amino acid.

2. CHROMOSOMES MUTATION

Chromosomes mutation involves changes in the structure of the chromosomes. During meiosis homologous chromosomes intertwine at several points called chiasmata and create opportunity for various changes on the chromatids leading to mutation.

TYPES OF CHROMOSOME MUTATION

a)      Deletion

b)      Duplication

c)      Inversion

d)      Trans location

e)      Non-disjunction

f)       Polyploidy

a). DELETION

This occurs when a portion of the chromosome breaks off and fails to reconnect to any of the chromatids, The result is the loss of genetic materials. Deletion can be caused by error in chromosomal crossover during meiosis. These causes serious genetic deceases

b). DUPLICATION

This occurs when a portion of the chromosome replicates itself adding extra length. The result is addition of a set of genes which is a duplication. This may result to over emphasizing of a trait in an organism.

c. INVERSION

This occurs when a middle piece of the chromosomes break and rotates at 180° and rejoins the chromatid. This has the effect of reversing the gene sequence.

d. TRANSLOCATION

This occurs when a portion of one chromosome breaks off and becomes attached to another chromatid of non-homologous pair. The result is transfer of genes from one pair of homologous chromosome to another.

e. NON-DISJUNCTION

This kind of chromosomal mutation is caused by addition or loss of one or more chromosomes. This occurs during meiosis where homologous chromosomes fail to separate. This results in some gametes having more chromosomes than others.

Example of non — disjunction

(i) DOWN’S SYNDROME  

This is caused by presence of an extra chromosome number 21 individuals with this defect have a total of 47 chromosomes they have

• Resistance to infection
• Mentally retarded
• Have thick tongue
• Short body

Also children of old parents (above 40 years woman and 55 man) have increased chance of Down’s syndrome.

(ii) KLINE FELTER’S SYNDROME

This is caused by failure of X chromosome to separate during the process of egg formation. An individual with this condition has two X chromosome and one Y chromosome (XXY). They are outwardly male but may also have female characteristics.

(iii) TURNER’S SYNDROME

This is an individual with 45 (44 + x 0) chromosome in a cell instead of 46 (44 +xx). Individual with this condition have one X and no Y i.e (XO) they individual is sterile and abnormally short female.

f. POLYPLOIDY

Occurs if the whole set of chromomes doubles after fertilization. where the spindle fail to be formed and the cell does not divide. It is rare in animals but common in plants

g. SICKLE CELL ANAEMIA

Sickle cell anaemia is an example of gene mutation. The normal haemoglobin is entirely replaced by abnormal haemoglobin known as haemolobin S. In sickle cell anaemia, the glutamic acid is replaced by another amino acid, the valine forming a haemoglobin s denoted by Hbs. Normal haemoglobin is denoted by HbA. Haemoglobin S begins to crystallize when Oxygen concentration falls and causes red blood cell to assume the shape sickle. Half the number of red blood cell is sickle.

GENETIC COUNSELING

Genetic information is used to advice couples who have hereditary disorders about chances of children inheriting the disorders. Genetic information could also be used in choosing marriage partners.

GENETIC ENGINEERING

This is the alteration of the structure of DNA by man. Genetic engineering enables man to carry out research.

• Manufacture protein (insulin)
• Improve animal and plant breeds
• Correct genetic disorders

Genetic engineering is the technique of changing the genotypes of an organism. It involves inserting genes from one organism into the chromosomes of other organisms. Once inserted the foreign genes work as if they were in the organism they were taken from.

APPLICATION OF GENETICS

1. Medicine

Genetics engineering has enabled biologists to program and make useful substance. For example the gene in man that produces insulin was inserted into escherichia colia for producing pure insulin in large quantities.

• Human growth hormone has also been made by using bacteria which the proper gene has been added.
• Also blood clotting factors such as fibrinogen needed by haemophiliacs are produce.
• Vaccines from viruses are produced.

2. Biological warfare

Genetic engineering can help humans to produce biological weapons i.e. Anthrax and Vibrio cholera

3. Agriculture

• It is common for farmers to select and plant seeds from the healthiest and high yielding varies of plants with the aim of improving desirable traits as high fruits and crop production.
• Also genetics has enabled the beginning of selective breeding. Selective breeding is the crossing of animals or plants that have desirable traits to produce offspring that have a connection of the parents’ desirable characteristics
• Also the knowledge of genetics developed in breeding which involves crossing relatively individuals to maintain desirable traits. The various breeds of cattle, dogs, pigeons, chicken and maize, sugarcane and goats are a result of in breeding

4. Genetic disorder

• Pregnant women can be informed about the deformation of the fetus
• It can help in the modification of disordered genes

Dangers of genetic engineering

1. The outcome of genetic engineering can be weird out of our imagination
2. Production of new pathogens accidentally or deliberately
3.