Question

You have been given a single virgin Drosophila female. You notice that the bristles on her thorax are much shorter than normal. You mate her with a normal male (with long bristles) and obtain the following F1 progeny: 1/3 short-bristle females, 1/3 long-bristle females, and 1/3 long-bristle males. A cross of the F1 long-bristle females with their brothers gives only long-bristle F2. A cross of short-bristle females with their brothers gives 1/3 short-bristle females, 1/3 long-bristle females, and 1/3 long-bristle males. Based on these observations, which of the following statements is incorrect?
A. The short-bristle mutation is associated with recessive lethality.
B. The short-bristle mutation is autosomal.
C. The short-bristle mutation is X-linked.
D. The short-bristle mutation is dominant.
E. The single virgin female is heterozygous for the short-bristle mutation.

Answer 1

Answer:

B

Explanation:

We never see short bristle males, suggesting some type of lethality. I.e. any males who inherit the mutation die before birth so we don't see the phenotype. This also hints that it could be X-linked.

Females can be short bristled, but males can't, as it is likely lethal. This suggests that having one copy of the short bristle trait without the long bristle trait is lethal (as males as XY and so only have one copy of the trait). The female then must be heterozygous for the short bristle trait (which also explains how in generation F2, long bristle males can be produced, as if she was homozygous males would all be short bristled, and therefore dead, so there would be no males.

Since the first short bristle female is heterozygous, the trait for short bristles must be dominant.

However, since evidence suggests the trait is X-linked, it cannot be autosomal, as suggested in B.