Question

In Drosophila, the gene for white (w) eyes is on the X chromosome and it is epistatic over brown and scarlet. White eyes can also result by crossing brown-eyed females (bw) with scarlet eyed males (st) (that is, when both mutant alleles are present).

A cross is made between white-eyed females with the genotype w//w (otherwise, homozygous for wild type alleles) and white-eyed males hemizygous for the wild type allele (w+ ).

List the expected: a) Genotypes of the P1 and P2 flies. b) F1 phenotypes and genotypes. 26 c) Expected F2 phenotypic ratios. Show your work.

CONCEPT PROBLEM How can the same phenotype stem from many different genotypes? Explain.

Answer 1

a) Male and female parents genotype is given X^WX^W and X^WY⁺ respectively.

b) A cross between the above-mentioned parents,

X^WX^W   X^WY⁺

^{F1 genotype,} X^WX^W X^WX^W   X^WY⁺ X^WY⁺

^{F1 phenotype = 1, all progeny will be white}

^{c) Parents,} X^WX^W X^WY⁺

^Gametes, X^W   X^W X^W Y⁺

^F2^generation,  X^WX^W X^WX^W   X^WY⁺ X^WY⁺

All F_²females and males will be white

The same phenotype can result from different genotypes because a single dominant allele of white is epistatic to other alleles in females and males. Moreover, males are hemizygous for the X-chromosome, so the only allele present on its X-chromosome can determine the X- linked phenotype.