The wild-type eye color in the fruit fly Drosophila melanogaster is dark red, as a result of a mixture of bright red and brown pigments. "Enzyme A" is encoded by the "a" gene, and is required to synthesize the bright red pigment. A lack of red pigment results in a somewhat brown eye color. You cross two fruit flies who are heterozygous for a recessive mutation that completely inactivates the "a" gene. What proportion of their offspring will have a recessive eye color phenotype?

1/4 or 25%

Explanation:

Let the wild-type eye color be represented by allele A. The mutant's eye color would be represented by allele a.

The genotype of fruit flies that are heterozygous for a recessive mutation that inactivates gene 'a' ( and therefore results in mutant eye color) would be Aa.

If Aa and Aa are crossed:

                      Aa     x     Aa

progeny:    AA    Aa    Aa    aa

The proportion of the offspring that will have a recessive eye color phenotype (aa) would therefore be 1/4 or 25%.

the cell cycle is the final answer! : )