In Drosophila, a cross (cross 1) was made between two mutant flies, one homozygous for the recessive mutation bent wing (b) and the other homozygous for the recessive mutation eyeless (e). The mutations e and b are alleles of two different genes that are known to be very closely linked on the tiny autosomal chromosome 4. All the progeny had a wild- type phenotype. One of the female progeny was crossed with a male of genotype b e/ b e; we will call this cross 2. Most of the progeny of cross 2 were of the expected types, but there was also one rare female of wild- type phenotype.

a. Explain what the common progeny are expected to be from cross 2.
b. Could the rare wild- type female have arisen by (1) crossing over or (2)nondisjunction? Explain.
c. The rare wild- type female was testcrossed to a male of genotype b e/ b e (cross
3). The progeny were 1/6 wild type, 1/6 bent and eyeless ,1/3 bent, and 1/3 eyeless. Which of the explanations in part b is compatible with this result?
Explain the genotypes and phenotypes of the progeny of cross 3 and their proportions.