Rediscovery of Mendel’s result:
- 1990 three scientists (deVries, Correns and von Tschermak) independently rediscovered Mendel’s result on the inheritance of character.
Chromosomal theory of inheritance:
- Proposed by Walter Sutton and Theodore Bovery in 1902.
- They work out the chromosome movement during meiosis.
- The behavior of chromosomes was parallel to the behavior of genes and used chromosome movement to explain Mendel’s laws.
- Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
- Chromosome and genes are present in pairs in diploid cells.
- Homologous chromosomes separate during gamete formation (meiosis)
- Fertilization restores the chromosome number to diploid condition.
- The chromosomal theory of inheritance claims that, it is the chromosomes that segregate and assort independently.
Experimental verification of chromosomal theory:
- Experimental verification of chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues.
- Morgan worked with tiny fruit flies, Drosophila melanogaster.
Why Drosophila?
- Suitable for genetic studies.
- Grown on simple synthetic medium in the laboratory.
- They complete their life cycle in about two weeks.
- A single mating could produce a large number of progeny flies.
- Clear differentiation of male and female flies
- Have many types of hereditary variations that can be seen with low power microscopes.
Linkage and Recombination:
- Morgan hybridized yellow bodied, white eyed females to brown-bodied, red eyed male and intercrossed their F1 progeny.
- He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from 9:3:3:1 ratio (expected when the two genes are independent).
- When two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type.
- Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage.
- Linage: physical association of genes on a chromosome.
- Recombination: the generation of non-parental gene combinations.
- Morgan found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) while others were loosely linked (showed higher recombination).
- The genes white and yellow were very tightly linked and showed 1.3 percent recombination.
- The genes white and miniature wing showed 37.2 percent recombination, hence loosely linked.
- Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome.
POLYGENIC INHERITANCE:
- Human have no distinct tall or short instead a whole range of possible heights.
- Such traits are generally controlled by three or more genes and are thus called polygenic trait.
- Besides the involvement of multiple genes polygenic inheritance also takes into account the influence of environment.
- Human skin color is another classic example of polygenic inheritance.
- In a polygenic trait the phenotype reflects the contribution of each allele i.e. the effect of each allele is additive.
- Assume that three genes A, B, C control the skin colour in human.
- Dominant forms A, B; AND C responsible for dark skin colour and the recessive forms a, b, c for light color of the skin.
- Genotype with dominant alleles (AABBCC) will have darkest skin color.
- Genotype with recessive alleles (aabbcc) will have lightest skin colour.
- Other combinations always with intermediate colour.
PLEIOTROPY:
- A single gene can exhibit multiple phenotypic expression, such gene is called pleiotropic gene.
- The mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contributes towards different phenotypes.
- Phenylketonuria a disease in human is an example of pleiotropy.
- This disease is caused due to mutation in the gene that code for the enzyme phenyl alanine hydroxylase.
- Phenotypic expression characterized by:-
- Mental retardation
- Reduction in hairs.
- Reduction in skin pigmentation.
SEX DETERMINATION:
- Henking (1891) traced specific nuclear structure during spermatogenesis of some insects.
- 50 % of the sperm received these specific structures, whereas 50% sperm did not receive it.
- Henking gave a name to this structure as the X-body.
- X-body of Henking was later on named as X-chromosome.
Sex-determination of grass hopper:
- Sex-determination in grasshopper is XX-XO type.
- All egg bears one ‘X’ chromosome along with autosomes.
- Some sperms (50%) bear’s one ‘X’ chromosome and 50% do not.
- Egg fertilized with sperm (with ‘X’ chromosome) became female (22+XX).
- Egg fertilized with sperm (without ‘X’ chromosome) became male (22 + X0)
Sex determination in insects and mammals (XX-XY type):
- Bothe male and female has same number of chromosomes.
- Female have autosomes and a pair of X chromosomes. (AA+ XX)
- Male have autosomes and one large ‘X’ chromosome and one very small ‘Y-chromosomes. (AA+XY)
- This is called male heterogammety and female homogamety.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]
