cn+ = wild type eyes (dull red), cn = cinnabar (bright red) eyes. w+ = wild type
ID: 12827 • Letter: C
Question
cn+ = wild type eyes (dull red), cn = cinnabar (bright red) eyes.w+ = wild type eyes (pigmented) w = white eyes (unpigmented).
Cinnabar and white are each recessive to their respective wild type allele. The cn allele is autosomal, while the w allele is X-linked. The white eye trait is epistatic to cinnabar, that is, all w w flies have white eyes, regardless of the cn genotype. A wild-type eyed female of genotype cn+ cn; w+ w is crossed to a cn+ cn; w Y white eyed male.
What progeny ratios for eye color will be produced among the male and female offspring?
Note: The wild-type allele of each gene is dominant only to its own mutant allele. Masking epistasis is NOT a form of dominance. Be careful to clarify your thinking about the difference between epistasis and dominance. Epistasis will not necessarily be defined on the exam.
Explanation / Answer
cn+ cn w+ w cn+ cn w Y cn+ cn+ 25% cn+ cn 50% cn cn 25% w+ w w Y 50% males have w+ Y 50% males have w Y 50% have w+ w females 50% have w w females male offspring 12.5% w+ cn+/cn+ 12.5% w+ cn/cn 25% w+ cn+/cn 12.5% w cn+/cn+ 12.5% w cn/cn 25% w cn+/cn total males: 37.5% cn+ w+ -> w.t 12.5% cn w+ -> cn 37.5% cn+ w -> white 12.5% cn w -> white ratio 3:3:1:1 female offspring 12.5% w+ w cn+/cn+ 12.5% w+ w cn/cn 25% w+ w cn+/cn 12.5% w w cn+/cn+ 12.5% w w cn/cn 25% w w cn+/cn total females: 37.5% cn+ w+ 12.5% cn w+ 37.5% cn+ w -> white 12.5% cn w -> white overall ratio: 4:3:1 50% of organisms will have white eyes 37.5% of organisms will be wild type 12.5% of organisms will have cinnabar eyes wikipedia: epistasis "In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic. Epistasis can be contrasted with dominance, which is an interaction between alleles at the same gene locus."
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