Question

In Drosophila, the X-linked recessive mutation vermillion (v) causes bright red eyes, in contrast to the brick-red eyes caused by the wild-type allele. A separate autosomal recessive mutation, brown (bw), causes flies to have brown eyes. Flies with both mutations have no eye pigmentation and are white-eyed. Determine the F1 and F2 phenotypic ratios for each sex from the following crosses:

(a) vermillion females X brown males

(b) wild-type females X white-eyed males

Answer 1

We have one X-linked and one autosomal recessive mutation, therefore we treat the two genes separately.

(a) For vermillion, v the female is XvXv that produces only Xv gamete and the male is X+Y, that produces X+ or Y gamete. In F1 progeny,

all females will be X+Xv and XvY will be male.  

F1 cross:
Vermillion X+Xv x XvY.
female gamete - 1/2 X+ and 1/2 Xv

male gamete- 1/2 Xv and 1/2 Y

F2 female - X+Xv and XvXv. Males - X+Y and XvY.

Now we get. F2 females-- 1/2 X+Xv x 1/4 ++ = 1/8 X+Xv++ normal wild type eyes.  

Then 1/2 XvXv x 1/4 ++ = 1/8 XvXv ++ vermillion eyes.

b) the cross is X+X+ x XvYbb++