CHROMOSOMAL THEORY OF INHERITANCE:
Why Mendel’s theory was remained unrecognized?
· Firstly communication was not easy in those days and his work could not be widely publicized.
· Secondly his concept of genes (or factors, in Mendel’s word) as stable and discrete units that controlled the expression of traits and of the pair of alleles which did not’ blend’ with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature.
· Thirdly Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.
· Finally he could not provide any physical proof for the existence of factors.
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.
o Chromosome and genes are present in pairs in diploid cells.
o Homologous chromosomes separate during gamete formation (meiosis)
o Fertilization restores the chromosome number to diploid condition.
o 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.
· 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.
· 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.
· 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:-
o Mental retardation
o Reduction in hairs.
o Reduction in skin pigmentation.
· 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.
Sex determination in birds:
Sex determination in Honey bee:
Genetic disorders is a disease that is caused by an abnormality in an individual's DNA. Genetic disorders are mainly of two types: Chromosomal disorder, cause by an abnormality in the chromosomes of individual. Mendelian disorder; caused by mutation or alteration in the single gene.
Gregor Jhon Mendel conducted hybridization experiments on garden peas(Pisum sativum) and proposed laws of inheritance.He proposed three laws of inheritance;law of Dominance, law of segregation, law of independent assortment.