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.
To determine the genotype of the red F2 individuals in cross 1, a test cross could be done. What should be the genotype of the flies used in the test cross? Assume the symbol b+indicates the wild type allele for the brown gene, while b indicates the mutation.
|D|| XbXb and/or XbY|
Recall that a test cross is used to confirm the genotype of an individual. The test cross is done with individuals that are homozygous recessive for the allele. When the individual with the unknown genotype is mated to the homozygous recessive individual, only two outcomes are possible: all the progeny are phenotypically dominant (indicated the unknown genotype is homozygous dominant) or half the progeny are phenotypically dominant and half are recessive (indicating the unknown individual was a heterozygote).