The reason for deviation from Mendel’s dihybrid cross in T.H. Morgan’s experiment is____________.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Linkage
In Morgan’s dihybrid crosses in Drosophila, the F2 ratio moved far from 9 : 3 : 3 : 1, and parental combinations were much more frequent than new ones.
Option (2) Linkage — Correct. The two genes were on the same chromosome (the X), so they did not assort independently. Morgan called this physical association linkage.
NCERT Extract:
Linkage and Recombination – “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). Morgan and his group knew that 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.”