Question

Assertion : When yellow bodied, white eyed Drosophila females were hybridised with brown-bodied, red eyed males; and F${}_1$ progeny were 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 in the offsprings are much higher than the non-parental type.

• A. Both assertion and reason are true and reason is the correct explanation of the assertion
• B. Both assertion and reason are true but reason is not the correct explanation of the assertion
• C. Assertion is true but the reason is false
• D. Assertion and reason both are false

Answer 1

The correct option is A Both assertion and reason are true and reason is the correct explanation of the assertion

In Morgan’s dihybrid cross in Drosophila, the genes for eye colour and body colour were found to be located on the same X chromosome.
When yellow bodied (y), white eyed (w) Drosophila females were hybridised with brown-bodied (y+), red eyed (w+) males; and F${}_1$ progeny was intercrossed, the F${}_2$ ratio deviated from 9:3:3:1. This is because the genes were situated on the same chromosome (X), on closer loci. The alleles did not get assorted independently. Hence, the proportion of offsprings with parental gene combinations was much higher than the non-parental types.