Chapter 3, Problem 6EQ

Let’s suppose that you have made a karyotype of a female fruit fly with red eyes and found that it has three X chromosomes instead of the normal two. Although you do not know its parents, you do know that this fly came from a mixed culture of flies in which some had red eyes, some had white eyes, and some had eosin eyes. Eosin is an allele of the same gene that has white and red alleles. Eosin is a pale orange color. The red allele is dominant and the white allele is recessive. The expression of the eosin allele, however, depends on the number of copies of the allele. When females have two copies of this allele, they have eosin eyes. When females are heterozygous for the eosin allele and the white allele, they have light-eosin eyes. When females are heterozygous for the red allele and the eosin allele, they have red eyes. Males that have a single copy of the eosin allele have eosin eyes. You cross the XXX red-eyed female with a white-eyed male and count the numbers of offspring. You may assume that this unusual female makes half of its gametes with one X chromosome and half of its gametes with two X chromosomes. The following results for the offspring were obtained:

	Females*	Males
Red eyes	50	11
White eyes	0	0
Eosin	20	20
Light-eosin	21	0

*A female offspring can be XXX, XX, or XXY

Explain the 3:1 ratio between female and male offspring. What is the genotype of the original mother, which had red eyes and three X chromosomes? Construct a Punnett square that is consistent with these data.