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 brown eyes fits with which of the following mechanisms?
Since the F1 progeny all are phenotypically similar to one parent (they have red eyes), and the F2 generation displays a phenotypic ratio of 3:1 red: brown, the brown trait must be recessive. Furthermore, since the pattern does not indicate a difference in heritablity in males and females, it must also be an autosomal trait.