Question

A test cross was performed between a red-eyed female fruit fly and a white-eyed male fruit fly. If some of the offspring have white eyes, what is the genotype of the female parental fruit fly?

Answer 1

The fruit fly with red eyes is heterozygous (Rr) for red eye color.#

Explanation:

The only way for a recessive trait can result from the test cross, is if the red-eyed fruit fly was heterozygous (Rr). The cross is between a red-eyed fruit fly and a white-eyed (rr) fruit fly. The only way an offspring can have white eyes, is if it inherited a recessive allele from both parents.

The gene for eye color in fruit flies is actually sex-linked on the X-chromosome. The Punnet square below represents a cross between a heterozygous red-eyed female and a white-eyed male. The symbol `W^+` is the dominant allele for red eyes, and the symbol `W` represents the recessive allele for white eyes. Only females can be homozygous or heterozygous for red eye color. The males have only one allele for eye color on their X-chromosomes.

Even though we might expect the cross to produce 50% red-eyed male and female fruit flies, and 50% white-eyed male and female fruit flies, it doesn't mean that that will happen. As long as one fruit fly has the white eye phenotype, it means that one parent (the female) is heterozygous.