In the fruit fly a gene locus for wing size is on the X chro

In the fruit fly, a gene locus for wing size is on the X chromosome. At that locus, allele B, for big wings is completely dominant to allele b, for small wings. Females have two X chromosomes (i.e., XX) while males are XY. The females (flies 3 and 4) are homozygous. Consider the following males, females & their phenotypes: a. Flies 2 and 3 mate. What fraction of the F1 generation has small wings? b. Flies 2 and 3 mate. What fraction of the F1 generation has big wings and is male? c. Flies 1 and 4 mate. What fraction of the F1 generation is female? d. Flies 1 and 4 mate. What fraction of the F1 generation has small wings? e. Flies 2 and 3 mate. Their F1-generation flies mate with each other to produce the F2 generation. What fraction of the F2 generation has small wings?

Solution

Genotypes:

Fly1: Male - XBY

Fy2: Male- XbY

Fly3:Female: XBXB

Fly4:Female: XbXb

(a)

XbY x XBXB

F1: XbXB, XbXB, XBY, XBY (all are big wings)

None of the F1 have small wings

(b) XbY x XBXB

All the F1 are big wings

100% of the males are of big wings

(c) XBY x XbXb

F1: XBXb, XBXb, XbY, XbY

50% of the F1 generation is female

(d) 50% of F1 generation has small wings

(e) XbY x XBXB

All the F1 are big wings

F1: XbXB, XbXB, XBY, XBY

F2: XBY x XbXB = XBXb, XBXB, XbY, XBY

25% of F2 have small wings