Question

Assertion :When yellow bodied, white-eyed Drosophila females were hybridised with brown-bodied, red-eyed males; and $F_1$ progeny was intercrossed, $F_2$ ratio deviated from 9 :3 : 3 : 1. Reason: When two genes in a dihybrid are on the same chromosome, the proportion of parental gene combinations are much higher than the non-parental type.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

When the genes for certain features (like yellow/white bodied or white/red eyed Drosophila) are located very close to each other on a chromosome, there are little chances of crossing over, giving higher parental combination. When the genes are on different loci on a chromosome, the distance between them considerable, there are higher chances of recombination, giving rise to mixed or non-parental features, for autosomes. However, since body color and eye color are present on X chromosomes, the deviation will be in relation to the gender in addition to recombination, which will not occur in males as they are hemizygous for the trait they carry.

So, the correct answer is 'Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion'