Sex-Linked Inheritance Problem Set

Problem 2: Test cross of a red-eyed female fly

Tutorial to help answer the question

A female Drosophila of unknown genotype was crossed with a white-eyed male fly, of genotype (w = white eye allele is recessive, w+= red-eye allele is dominant.) Half of the male and half of the female offspring were red-eyed, and half of the male and half of the female offspring were white-eyed. What was the genotype of the female fly?

Tutorial

Possible Female Genotypes

Genotypes and phenotypes of offspring if the female is homozygous

All of the male and female offspring of a homozygous female would be red-eyed.

Genotypes and phenotypes of offspring if the female is heterozygous

Half of the male and half of the female offspring of a heterozygous female would be white-eyed; the other half would be red-eyed.
Therefore the female must be a heterozygous, X^w+ X^w.

Summary

We had a cross between a female Drosophila of unknown genotype with a white-eyed male fly. The result of the cross was that half of the male and half of the female offspring were red-eyed, and half of the male and half of the female offspring were white-eyed.
What was the genotype of the female fly? The female fly had to be heterozygous, X^w+ X^w.

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Updated: July 15, 1999
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