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

a. We have vermillion females crossed with brown eyed males: vv X bwY

F1 generation:

	v	v
bw	vbw (white, female)	vbw (white, female)
Y	vY (vermillion, male)	vY (vermillion, male)

Phenotypic ratio: 1:1 (white female : vermillion male)

F2 generation: vY x vbw

	v	Y
v	vv (vermillion, female)	vY (vermillion, male)
bw	vbw (white, female)	bwY (brown, male)

The phenotypic ratio for the F2 generation is 1:1:1:

b. We have wild type females crossed with a white-eyed male. This is assuming that in the X chromosome of the male, both mutations are present.

F1 generation: XX X wY

	X	X
w	Xw (normal, female)	Xw (normal, female)
Y	XY (normal female)	XY (normal, male)

F2 generation: Xw X XY

	X	w
X	XX (normal, female)	Xw (normal, female)
Y	XY (normal, male)	wY (white eyed male)

The phenotypic ratio is 1:2:1