In the fruit fly, Drosophila melanogaster, many different genes influence eye color. Mutations in any one of these genes will change the color of the eyes. Normally, flies have brick red eyes (this is called the wild type condition). The inheritance patterns of these variations has been extensively studied. The results of some crosses are described below.
Wild type flies were crossed with brown eyed flies. The F1 progeny all had red eyes. When the F1 were crossed with each other, 3/4 of the F2 had red eyes and 1/4 had brown eyes.
Wild type females were crossed with white eyed males. All of the F1’s had red eyes. In the F2 generation, all of the females had red eyes, while half of the males had red eyes and half had white eyes.
Brown eyed flies were crossed with scarlet eyed flies (scarlet is known to be an autosomal recessive trait). All of the F1 progeny had red eyes. The F2 progeny showed a ratio of 9:3:3:1 red to brown to scarlet to white.
Another mutation affecting the eye, called Bar, does not affect color. Instead, it affects the shape of the eye, changing it from the normal round phenotype to an elongated, oval shape. Wild type flies were crossed with Bar eyed flies. Half of the F1 progeny had Bar shaped eyes and half had wild type eyes. When the wild type F1 flies were crossed with the Bar eyed F1 flies, the F2 generation showed the same phenotypes as the F1 generation: half were wild type, half had Bar eyes.
The inheritance pattern of white eyes fits with which of the following mechanisms?
The pattern seen is consistent with a sex-linked recessive trait. Genotypically, the F1 generation consists of all females heterozygote for the trait (as they inherit one X from the male parent and one from the female parent) and all males will be hemizygous for the normal (wild type) allele (as they can only inherit the X from the female parent). In the F2 generation, the F1 females will contribute the wild type allele to half the male progeny and the white allele to the other half, resulting in the phenotypes reflecting the genotypes. However, the male F1’s only have a wild type allele, which will be passed on to the females in the F2 generation, so all these flies will have red eyes, regardless of what allele they inherit maternally. To extend your knowledge and understanding, determine what would happen if the reciprocal cross was done (white eyed females crossed with red eyed males in the parental generation).