Question

You cross a Drosophila female with kidney-shaped and brown eyes with a male that has wildtype eye shape and color. Kidney-shaped eyes are found in females that are heterozygous for the bar eyed mutation. Bar eyed females are homozygous for the bar eyed mutant allele, whereas bar eyed males are hemizygous for the mutation. Brown eyes are an autosomal recessive mutation. 1/2 of the resulting F1 has brown eyes!

1. What proportion of the F1 will be females with wildtype eye shape and color?
2. Would you expect to see bar eyed females in the F2? Justify your answer.

Answer 1

In the F1 generation, 25% of females will have wildtype eye shape and color. In the F2 generation, bar-eyed females are expected because F1 heterozygous females can produce homozygous or heterozygous daughters for the bar mutation when crossed with a bar-eyed male.

Step-by-step explanation:

The question involves understanding the X-linked inheritance in Drosophila melanogaster (fruit flies), specifically regarding eye shapes and colors. Kidney-shaped eyes are known to be caused by the bar mutation, which is X-linked, and brown eyes are due to an autosomal recessive trait.

F1 Generation: Given that half of the F1 offspring have brown eyes (the autosomal recessive trait), the female parent must be heterozygous for this trait (Bb) as brown is recessive. Therefore, the brown-eyed F1 individuals (50%) must be both male and female since the trait is autosomal. For females to exhibit wildtype eye shape (not kidney-shaped), they must not carry the bar allele at all, which indicates that all F1 males will have wildtype eye shape since they have their mother's X chromosome, but only half of the F1 females will have wildtype eye shape since they will be heterozygous for the bar mutation (XBXb).

Since wildtype eye color (red) is dominant, all F1 offspring will display red eyes if they do not carry the brown eyes mutation (bb). Since half the F1 offspring have brown eyes (bb), the other half will have red eyes, and this applies to both males and females.

Therefore, the proportion of the F1 females with wildtype eye shape and color is calculated by considering only the half that have wildtype shape (heterozygous for bar mutation) and the half that do not have brown eyes (B-). This gives us ½ female wildtype shape × ½ female non-brown eyes, which is ¼ or 25% of the F1 females with wildtype eye shape and color.

F2 Generation: To see bar-eyed females in the F2 generation, you would need a bar mutation allele from both parents. Since the F1 females are heterozygous for the bar mutation, they can pass on the mutation to their offspring. Therefore, yes, you would expect to see bar-eyed females in the F2 generation because F1 heterozygous females (XBXb) could have daughters who are either heterozygous or homozygous for the bar-eye mutation when crossed with a bar-eyed male (XbY).