Assertion (A) : In Thomas Hunt Morgan’s dihybrid crosses in Drosophila, the F2 generation showed a deviation from the expected 9:3:3:1 ratio, which is typically observed when two genes assort independently.
Reason (R): Morgan discovered that when genes are located on the same chromosome, they tend to be physically linked and do not assort independently, leading to a higher proportion of parental gene combinations compared to non- parental ones.

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.

The genes were located on the X chromosome (Section 4.4) 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.