ALBIO5: GENETICS

This unit dwells on how genetics influences mankind and how it determines the sex of human beings

Genetics

This is the study of the mechanism of heredity or inheritance whereby traits or characteristics are passed on from generation to generation.

Some traits of importance in genetic.

  • Hair, skin, eye colour
  • Curly or straight hair
  • Height
  • Boldness
  • Form earlobe
  • Intelligence
  • Ability to roll the tongue
  • Colorblindness
  • Ability to test the chemical phenylthiocarbamide
    • Heamophilia
    • Blood gps A, B, AB, O
    • Druping eyelids
    • Ability to produce insulin.

     

    Practical application of genetic

    • Animal breeding. Farmers select animals with good characteristics for breeding purposes i.e. artificial selection and these pass on their traits to their off springs.
    • Farmers use the mating system to get animals with good traits and there are 2 types of mating systems i.e. out and in breeding.
    • Outbreed involves mating the male and female animals which are unrelated and belong 2 different breeds while inbreeding involves mating of male and female animals which belong to the same breed and are related.

In big production, the farmer has to look at pigs 4 pork and the one for beacon and the dual purposes ones.

In cattle production, dairy, beef and dual purpose ones.

Poultry production, there are the layers and broilers

MENDELIAN GENETIC

The father of genetic is Gregor Mandel. He is known for his breeding experiments. He used the gander ped (pisum satisum). He studied 7 characterisitcs of this pea and each characteristics had 2 traits namely;

  • Seed shape
  • See colour
  • Flower colour
  • Pod shape
  • Pod colour
  • Flower position
  • Plant height

Its procedure

  • He selected pure breeding plants with different expressions of a particular characteristic like plant height i.e. shortness and tallness.
  • He crossed the parent plants by transferring the pollen of one plan (made) to the stigma of the 2nd plant (+0).
  • He then removed the anthers of the 2nd plant and covered the flower with fine muslin bags to make sure that only pollen from the required plant was transferred.
  • He collected seeds from the female parent plant and planted them. These gave rise to the 1st offspring generation (F1).
  • He recorded the characteristics of each of these hybrid plants
  • 2 plants from the F1 generation were then crossed. Since they were from the same generation, it was a self cross.
  • The pea seeds were collected from theF1 +0 plants and were grown they gave rise to the 2nd offspring generation (F2). He then recorded the characteristics of the plant.

Using these breeding experiments, mandel observed the inheritance of parents characterisitics through 2 generations of offsprings.

Its results

  • All the F1 offsprings were tall.

All the F2 offsprings had a ratio of tallness; shortness being 3:1

Mendel’s laws

Mendel proposed 2 laws to explain the behaviour of genes the he called factors at that time.

Mendel’s 1st law

This is the law of segregation. It states that factors are present in pairs in somatic cells. When an organism forms gametes, the factors separate so that each gamete contains only one factor of a pair. It is now known that genes are arranged on chromosomes which are present in homologous pairs in the somatic diploid cells and when the haploid gametes are formed, homologous chromosomes are separated by meiosis so that each gamete contains one chromosome of each pair. Genes present in pairs in somatic cells are present singly in gametes. The pair of genes has alleles they are genes which occupy the same position in homologous chromosomes. The position is known as locus.

Mendel’s 2nd law.   

This is the law of independent assortment. This states that when more than one pair of factors are considered. Each factor of a pair combines with any other factor of another pair as long as the genes aren’t linked i.e. on the same chromosome. This law leads to dihybrid inheritance which is the study of inheritance of 2 pairs of genes.

MONOHYBRID INHERITENCE

This is the study of the inheritance of a single pair of contrasted traits and the following way illustrates the correct way to describe a monohybrid cross

 

Let     T represent the allele for fallness in a pea plant (dominant) and

t represent the allele for shortnes (cressessive) in a pea plant.

Parental phenotypes:   Pure breeding Tal                  x        Pure breeding short

geno

F1 selfed

F1 phonotypes:    Heterozygous Tall                  X       Heterozygous tall

 

F1 genotypes                          Tt                                   Tt

gam

F2 phenotypes                       Tall    Tall                  Tall    Short

 

3:1 is the typical unit of inheritance for a given trait.

Gene

This is the basic unit of inheritance for a given trait.

Alleles

This is a pair of genes which occupy the same locus on homologous chromosomes and determine a particular characteristic e.g. T and T are alleles.

Homozygous

This is a diploid condition where both alleles are indential e.g. TT and tt.

Phonotype

This is the physical or chemical expression of a characteristic e.g. tallness.

Genotype

This is genetic expression of a characteristic in terms or alleles.

If a pure breeding peaplant with tound seeds is crossed with a pure breeding peaplant with wrinkled seeds, what would be the phenotypes of the F1 and F2 generations?

Let R represent the allele for roundness in seeds (dominant).

Let r represent the allele for wrinkledness in seeds (recessive).

olx

Parental phenotypes    Pure breeding           Pure breeding

Wrinkled                    Round

types

Gametes

Random fertilization

F1 genotypes

                                          Rr      Rr                     Rr    Rr

F1 phenotypes                        All round.

self

hetero

F2 genotypes                RR    rr                     Rr       Rr

F2 phenotypes      Round   wrinkled        Round    Round

 

If a homozygous dominant pea plant with green pods is crossed with a homozygous recessive plant with yellow pods. What would be the F1 and F2 phenotypes and genotypes.

 

Let G be allele for green pods.

g be the allele for yellow pods.

 

F1 phenotypes              Pure breeding               Pure breeding

Homozygous                 Homozygous yellow

gametes

F1 genotypes                Gg      Gg                                  Gg       Gg

F1 phenotypes               All green

 

Examples of monohybrid inheritance in man include cystic fibrosis, chondrodrytropic dwarfism (Achondroplasia) – caused by a dominant gene.

Albinism is caused by a recessive gene.

Hybrid inheritance

The study of the inheritance of two pairs of contrasted characters.

A pure breeding peaplant which is tall and has green pods is crossed with a pure breeding peaplant which is short and has yellow pods. What is the F1 and F2 genotypes and phenotypes.

Let     T represent the allele for tallness dominant.

t represent the allele for shortness recessive.

G represent the allele for green pod dominant.

g represent the allele for yellow pod recessive.

.

O tall green                 O short yellow

+

TTGG                            ttgg

 

TG    TG                   tg  tg

 

F1 generation Tt Gg      TtGg    GtGg      TtGg

F1 phenotype All Tall Pea Plant with green pods.

Key for the phenotypes * Tall greenO Tall yellow

Short green

Short yellow.

Use a punnett square to obtain the F2 phenotypes and genotypes.

tg

Phenotype ratio of F2 is:

9 tall green: 3 short green: 3 tall yellow: 1 short yellow.

(Typic ratio for dihybrid inheritance)

BACKCROSS

backcross

 

This is crossing of one of an organism with one its parents.

TEST CROSS

test

This is the crossing of an organism having an unknown genotype with a homozygous recessive organism in order to determine the unknown genotype.

It the organism with the unknown genotype is homozygous dominant, the all the offsprings will have the dominant pherotype but if the organism with the unknown genotype is hetozygous, then some of the offsprings will have the dominant phenotype and others will have the recessive phenotype eg in the fruit fly (drosophila), long wing is dominant to vestigial wing is homozygous fruitfly with long wings is crossed with one with vestigial wings, then all the offsprings will have long wings.

CODOMINANCE

This is a situation in which both alleles are expressed equally in the phenotype of the heterozygote. E.g. alleles for blood group A and B, alleles for coat colour in cattle, letters used for codominance are different and are both capital.

INCOMPLETE DOMINANCE

This is the situation where the teterozygote exhibits a phenotypic intermediate between the pherotype of the two alleles g colour of flowers in snap dragon. The colours of the flowers are red and white and the heterozygotes are pink.

E.g.

Let W be the allele for White flower.

Let R be the allele for Red flower.

pink

Example of testcross

Crossing a homozygous drosophila with long wings with one with vestigial wings.

Let V the allele for long wings

Let v be the allele for vestigial wings

qm

Example of codominance

Crossing a white but with a black cow

Let W be allele for white coat.

Let B be allele for black coat.

white

BIOCHEMICAL BASIS OF DOMINANCE

Many genes work by controlling chemical reactions by means of enzymes therefore, on a biochemical basis, a dominant allele is one which usually codes for a functional product such as an enzyme which even when present in only one copy i.e. in the heterozygous condition is capable of synthesizing enough product to produce a phenotype identical to that produced when there are 2 copies of the dominant allele in the cell.

In contrast, the recessive allele usually the synthesis of a non directional product and its phenotype can only be expressed when there is no dominant allele present.

Codominance

A situation where both alleles at a locus produce functional products and it the 2 alleles code for different products then both products will appear in the heterozygote.

Incomplete dominance

A situation where the alleles produce insufficient products.

Sickle cell anaomia

It is a gentic disease of the humans and provides an example of codominance dominance, and incomplete. The alleles involved affect the hemoglobin.

gen

  1. With regard to anaemia, the HbA allele is dominant to the Hbs
  2. With regard to the red blood cell, the HbA and Hbs alleles are incompletely dominant to each other.
  • With regard to the hemoglobin, the HbA and Hbs alleles are codominant.

If a normal person who is a heterozygote marries another heterozygote. What is the offspring genotype and phenotype.

Complementary gene action

 

Involves the interaction between genes at different loci. In monohybrid and dihybrid crosses, there is no interaction between genes at different loci instead each gene locus controls a separate character.

Example.

It is possible to have 2 different breeding varieties of the sweet pea both of which have while flowers. This is because flower colour in sweet pea is controlled by two loci. If the 2 varieties are crossed, the F1 generation will all have purple flowers.

If F1 is selfed, F2 generation has phenotypes in the ratio a purple: 7 white.

code

Compound x and compound y are both colourless. Conversion of x into y is catalyzed by enzyme E2. The synthesis of functional enzymes E1 and E2 is directed by the alleles C and P respectively. Recessive alleles C and P code for the production of non-functional enzymes so, A plant with genotype CCpp would be white since it can’t produce function enzyme E2 and can’t convert y to the purple pigment 2. it can only manage to convert x to y.

A sweet pea with genotype ccPP would have white flowers since it can’t produce functional enzyme e, hence can’t produce y which is essential for the synthesis of the purple pigment.

Let C represent the allele for presence of a pigment (dominant)

Let c represent the allele for lack of a pigment (recessive)

Let P represent the allele for purple flower (dominant)

Let p represent the allele for white flower (recessive)

plu

f1

Use the punnett square to obtain the F2 genotypes and phenotypes

purp

Phenotypic ratio is 9 purple: 7 white.

EPISTATIC

Epistatis is defined as the interaction between 2 different genes that aren’t alleles so that an allele of one of them interferes and inhibits the phenotypic expression of another gene. E.g. in the sweet pea, the gene for pigment formation is epistatic over the one for flower colour.

Gene C is epistatic over gene P. the gene which hinders the expression of another is known as the epistatic gene. The gene which doesn’t allow itself to be expressed is referred to as the hypostatic gene.

Examples

Coat colour in mice: wild mice have different coat colours. The wild mice can have agouti coat colour. They can also have black fur.

The gene for agouti is dominant to the gene for black fur.

Another independently inherited gene is required for the synthesis of the hair pigment. This gene C is responsible for the formation of the hair pigment while another gene c is responsible for lack of the hair pigment. Gene  C is dominant over gene c.

Let A represent the gene for agouti fur (dominant)

Let a represent the gene for black fur (recessive)

Let C represent the gene for pigment formation (dominant)

Let c represent the gene for lack of the pigment.

aguti

Using the punnett square

alb

F2 phenotypic ratio: 9 Agouti: 3 black fur: 4 Albino

Lethal alleles

E.g. inheritance of fur colour in mice. The allele for yellow fur (Y) is dominant to the allele for grey fur (y). If a pair of yellow mice are mated the result is always the same, two thirds of the offsprings are yellow, 1/3 grey i.e. yellow to grey occur in a ratio 2:1. The fact that both offsprings include both types of fur colour means that both parents must be heterozygous (Yy). But genetic crossing give a ratio of 3:1 in F1. The simplest explanation to this is to postulate that individuals homozygous for the yellow allele (YY) dies before birth therefore the alleles YY represent a lethal combination of alleles. As shown below, the death of such individuals in the embryonic state would have the effect of removing a quarter of the offspring from the litter. The dead embryos would represent 1/3 of the potential yellow offsprings thus reducing the proportions of yellow offspring ¾ to 2/3.

yy

This hypothesis is correct and is confirmed by two further observations.

  1. Crossing yellow with yellow never produces exclusively yellow offspring i.e. it is impossible for a pair of yellow mice to breed true,
  2. This is explained only by assuming that yellow mice are always heterozygous and a living homozygous yellow mouse is an impossibility.
  3. The expected number of dead embryo ie ¼ of the total offspring, are found in the uteri of yellow mothers who have been mated with yellow males. Such uteri are not found in grey mothers (or of yellow males mated with yellow mother mated with grey males) because none of the offspring from such matings would be YY.

This evidence shows that YY combination is lethal i.e. the U allele not only affects colour but also viability. It is both dominant and recessive i.e. as allele controlling fur colour, it is dominant and as allele affecting viability, it is recessive exerting lethal affects only when in homozygous state.

Lethal alleles are known to exist in a wide range of organisms including humans. The lethal allele can exert partial dominance in certain cases when it can be detected from the heterozygous condition. Though this is not lethal, it can have an observable effect on the individual’s phenotype.

LINKAGE

This is when genes are situated on the same chromosome and all genes on the same chromosome form a linkage gp and usually pass into the same gamete and are inherited together. As a result of this genes belonging 2 the same linkage gp usually don’t show independent assortment so they fail to produce the expected 9:3:3:1 ratio in dihybird inheritance E.g. the genes for body colour and wing length in drosophila are linked. The genes for grey body and the gene for long wing are dominant over the gene for black body and vestigial wing.

If a pure breeding grey bodied long winged drosophila is crossed with a black bodied vestigial wing drophilla, the expected F2 phenotypic ratio would by 9:3:3:1. However, this result is not obtained instead, the F2 show an ~ 3:1 ratio of the parental phenotype because they are linked (genes) i.e. they are found on the same chromosome. This can be represented diagrammatically.

 

Let G represent the gene for grey body (dominant)

Let g represent the gene for black body (recessive)

Let L represent the gene for long wing (dominant)

Let l represent the gene for vestigial wing (recessive).

Let c represent the gene for lack of the pigment.

vl

In practice though, this 1:3 ratio is never produced because total linkages is rare. So most breeding experiments involving linkage produced approximately equal numbers of the parental phenotypes and a significantly smaller number of phenotypes showing new combinations of characteristics also in equal numbers. These later phenotypes are described as recombinants.

Therefore recombinants are phenotypes produced after crossing over has taken place. When 2 or more genes are linked, then phenotypes with new gene combinations i.e. recombinants occur loss frequently than the parental phenotypes.

SEX DETERMINATION


This is the method by which sex is determined. One of the most common methods is by means of the sex chromosomes. Sex chromosomes are also known as heretozomes. The sex chromosomes are chromosomes associated with sex determination and are 2 types i.e. the X and Y chromosomes.

All the other chromosomes which aren’t sex chromosomes are known as autosomes.

In the production of gametes, the sex chromosomes segregate in the normal mendelia fashion. In the mammal, each ovum contains on x chromosomes in the females. In the males, one half of the sperms contains an x chromosome. The other half contains a y chromosome.

Probability of producing a male and female child

girl

Difference between sex linked genes and sex limited genes.

Sex linked genes are those that are carried on sex chromosomes. The characteristics of these can appear in both males and females whereas sex limited genes are those characteristics expressed in only one of the sex

Crossover value

This shows the frequency with which recombination occurs. It demonstrates that linked gas are arranged linearly along the chromosome and it reflects the relative positions of gene on a chromosome.

High crossover values indicate that crossing over has occurred and that the 2 genes are situated abit far from each other.

Low 2 genes are near each other.

High crossover should be above 50% and low below 50%.

VARIANCE

 

 

These are the differences in characteristics between members of the same species.

Variations are due to:

  • Environment
  • Genotypes

Genetic variations are due to new combinations of genes during meiosis. The new combinations during meiosis are brought about by:

Segregation of alleles so that each gamete has one allele for a particular gene locus.

  • Random orientation of bivalents in metaphase 1 and random orientation of chromosomes in metaphase 2 and this leads to independent assortment when they separate in anaphase 1 and anaphase 2. This leads to random assortment of the material and paternal chromosomes in the gametes.
  • Crossing over in meiosis leads to new combinations of genes which leads to variation of characteristics. These inherited variations are the raw materials on which natural selection acts and they cause the evolution of species.

When one studies the phenotypic differences in any population of species one sees 2 types of variation.

  • Continuous variation
  • Discontinuous variation

Continuous variation

This is when there is a gradual change in a characteristic in a population of species from one extreme to the other and there is no sharp clear-cut difference between the individuals therefore there are intermediate. E.g. height, weight, intelligence, skin colour in a race, ear length, yield of milk and meat in cattle, number of maize grains on a maize cob, shape and size of organs.

When plotted, each characteristic gives a smooth bell shaped curve known as the normal distribution curve or Gaussian curve.

hill

The characteristics which show continuous variation are brought about by:

  • Action of many genes i.e. polyenes and each of those has a small after and causes a gradual change in the characteristic of a species.
  • The characteristics are present in all members of a population.
  • The characteristics are measurable i.e. quantitative.
  • The characteristics are influenced in the environment.

Discontinuous variation

This is when there are clear cut differences in the characteristics of a population of a species and there are no intermediates. E.g. sexes, blood gps in man, normal and vestigial wings in man, light in peas, tongue rolling seed shape is peas, eye colour in drosophila, sickle cell anaemia, albinism, polydactly, eye colour in certain races etc. when plotted, such characteristics don’t give a bell shaped curve. A bar graph is used when plotting. These characteristics are caused by:

  • Different characteristics of a gene.
  • These characteristics are either absent or present in an individual in a population.
  • These characteristics aren’t measurable but are qualitative.

These characteristics are largely not influenced by the environment

 

 

 

 

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