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

Answer=if we have one X-linked & one autosomal then it is easier to treat the two genes separately & combine the results at the end.

(a) vermillion females X brown males

for vermillion,

v is female is =XvXv

male= X+Y

parent=   vermillion females X brown males

genotypes = XvXv X X+ Y

gametes= Xv Xv X+ Y

F1 generation= XvX+ XvX+ XvY XvY

=(female) (female) (male) (male)

all F1 females are XvX+ & all males are XvY

brown +b X +b

both produce + & b gametes which combine to produce ++ +b b+ & bb

Now combine

F2 females will be 1/2X +Xv X 1/4 ++ = 1/8 X+Xv++ normal wild type eyes

and 1/2 X+Xv X 1/2 +b =1/4X+Xv+b normal wild type eyes .

finally

1/2X+Xv X 1/4bb = 1/8 brown eyes.

then 1/2 XvXv X1/4 ++ = 1/8XvXv++ vermillion eyes: 1/2 XvXv X1/2b+ =1/4XvXv+b vermillion eyes and 1/2XvXv X1/4 bb =1/8 V =XvXvbb white eyes.

F2 females 1/2 X+Y X 3/4 ++ or +b =3/8 X+Y++/X+Yb+ brown eyes,1/2X+Y X 1/4bb= 1/8 X+Ybb brown eyes.

1/2XvY X3/4 ++/+b =3/8 XvY++ or +b vermillion eyes and 1/2 XvY X 1/4bb =1/8 XvYbb white eyes.

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