Question

The yellow mutation is an X-linked recessive mutation in Drosophila, causing yellow body color. A yellow-bodied female is crossed to a male with wild type body color to produce F1 progeny. The F1 female progeny and F1 male progeny are then crossed to each other to produce F2 progeny. Among the F2 females

A. all will be yellow body color.

B. 3/4 will be yellow body color.

C. 1/2 will be yellow body color.

D. 1/4 will be yellow body color.

E. none will be yellow body color.

Answer 1

Option C is the correct answer.

Let suppose, yellow body female alleles to be and wild type body color male alleles to be XY.

F1 Cross:

Yellow bodied female * Wild type body color male

* XY

	X	Y
	X( Normal, female)	Y (Normal, male)
	X(Normal, female)	Y (Normal, male)

In F1 progeny all are wild type or normal but none of them are yellow bodied because it is a recessive mutation it needs to mutant alleles to produce yellow body.

F2 Cross:

F1 female (X) * F1 male (Y)

X * Y

		Y
X	X (Normal, female)	XY (Wild type, male)
	(Yellow body, female)	Y (Normal, male)

In F2 we got

1/4 - Normal, female, 1/4 - Yellow body, female, 1/4 Wild type, male and 1/4 Normal, male.

So, in the question we have been asked among the F2 females we have only 1/2 yellow body color females. Since, we have only 2 females in F2 progeny out of the two we have one yellow body female that becomes 1/2.