In fruit flies, brown eyes can be caused by the recessive mutations pn (prune),
ID: 48304 • Letter: I
Question
In fruit flies, brown eyes can be caused by the recessive mutations pn (prune), bw (brown) or ry (rosy), which are on the X chromosome, second chromosome, and third chromosome, respectively. Wild-type alleles for all three genes must be present for eyes to be red. Suppose that two brown-eyed flies are crossed and their progeny are either brown-eyed males or red-eyed females. Which mutation is responsible for the brown eyes in the parental female?
A. pn
B. bw
C. ry
D. either bw or ry
E. any of the three — pn, bw, or ry
Explanation / Answer
Only pn (prune) mutation is present in X chromosome.
Let the genotype for wild type alleles be X, Bw and Ry.
The genotype for brown eyes (which are caused by recessive mutations) can be either ryry, bwbw, or XpnXpn(in females) and XpnY (in males).
Now, in the given cross , the entire female progeny after the cross has red eyes. So, the genotype of progeny females should be X_ / Bw_/ Ry_
The cross is (female) XpnXpn / BwBw / RyRy crossed with (male) XY / bwbw / ryry
Checking the results, Xpn gene seems to responsible for brown colour in the parental female. So, the mutation responsible for brown colour in parental female is prune, pn (option A).
Also, we can see that the mutation responsible for brown colour in parental male is either bw or ry.
The parental male