The reason for deviation from Mendel's dihybrid cross in T.H. Morgan's experiment is____________.

Linkage

The correct answer is Option (2) –Linkage

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas.  For example 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).

He observed that the two genes did not segregate independently of each other while he crossed brown bodied, red eyed males with yellow bodied, white eyed females. After selfing F1, he observed that these genes were not independently assorted and the F2 phenotypic ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes get assorted independently).

It was observed that in the F2 generation, there were more offspring of parental type (98.7%) and less percentage of offspring of recombinant type (1.3%). His observations further provided a concrete evidence of the fact that certain genes were sex-linked.

Morgan also coined the term linkage to describe this physical association of genes on a chromosome. Morgan and his group knew that the genes were located on the X chromosome and saw quickly 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 attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations .