How many back-crosses would be necessary to purge 99.9999% of the inferior genes from the white-eyed fly?
Often geneticists want to change one allele in an out-crossing organism while keeping the rest of the genome the same. For example, they might wish to take a specially designed stock of flies and alter the eye color from red to white. Suppose that the white-eye allele is dominant, meaning that flies with one or two white-eye alleles will have white eyes. One procedure used is to take a white-eyed fly and cross it with the red-eyed stock. The white-eyed offspring are then considered to be the first generation, and are crossed with the red-eyed stock. Their white-eyed offspring are considered to be the second generation, and are again crossed with the red-eyed stock, and so forth. The special red-eyed stock is homozygous for the desirable allele A at some other locus, but the white-eyed fly is homozygous for the inferior a allele at that locus.