Compare in any three ways the chromosomal theory of inheritance as proposed by Sutton and Bovery with that of experimental results on pea plant presented by Mendel.

OR

(a) Explain linkage and recombination as put forth by T.H. Morgan based on his observations with Drosophila melanogaster crossing experiment.

(b) Write the basis on which Alfred Sturtevant explained gene mapping.

i. Mendel deduced that genes come in pairs and are inherited in distinct units, one from each parent and tracked the segregation of genes from the parents and their expression in the offspring as dominant or recessive. But he didn’t find out how characteristics are sorted and combined on cellular level.
Sutton and Bovery suggested that chromosomes bear the hereditary material.

ii. Mendel knew nothing of the chromosomes and meiosis. Sutton and Boveri in order to explain the law of independent assortment suggested that different genes were in different chromosomes completely independent of one another.

iii. Sutton and Boveri argued that the pairing and segregation of a pair of chromosomes will lead to the segregation of a pair of factors they carry. Sutton united the knowledge of chromosomal segregation with Mendel’s principles and called it “Chromosomal theory of inheritance”.

OR

(a)
Linkage is the term used to describe the physical association of genes on a chromosome.
Recombination is used to describe the generation of non-parental gene combinations.

•T.H. Morgan and his colleagues used Drosophila melanogaster to study linkage. Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males 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 the 9:3:3:1 ratio (expected when the two genes are independent). They knew that the genes were located on the X chromosome.

•They observed that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.

•Morgan and his group also 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).

(b).

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.