Chapter 12, Problem 20P

In Drosophila, the gene for cinnabar eye color is on chromosome 2, and the gene for scarlet eye color is on chromosome 3. A fly homozygous for both recessive cinnabar and scarlet alleles (cn/cn; st/st) is white-eyed.

a.	If male flies (containing chromosomes with the normal gene order) heterozygous for cn and st alleles are crossed to white-eyed females homozygous for the cn and st alleles, what are the expected phenotypes and their frequencies in the progeny?
b.	One unusual male heterozygous for cn and st alleles, when crossed to a white-eyed female, produced only wild-type and white-eyed progeny. Explain the likely chromosomal constitution of this male.
c.	When the wild-type F females from the cross with the unusual male were backcrossed to normal cn/cn; st/st males, the following results were obtained: wild type 45% cinnabar 5% scarlet 5% white 45%

Diagram a genetic event at metaphase I that could produce the rare cinnabar or scarlet flies among the progeny of the wild-type F₁ females.