The fruit fly Drosophila melanogaster has a haploid chromosome number of n = 4,

Question

The fruit fly Drosophila melanogaster has a haploid chromosome number of n = 4, and 2n = 8. Chromosome IV in this species is a tiny autosome. Flies trisomic for chromosome IV are fertile and have no apparent defects.

The eyeless (ey) and gawky (gw) loci are tightly linked on chromosome IV. Loss-of-function eyand gw alleles are recessive to ey and gw, respectively. Flies homozygous for ey lack eyes, and flies homozygous for gw have disrupted circadian rhythms.

Consider a male fly trisomic for chromosome IV, with each of the three chromosome copies bearing different allele combinations for these two loci:

If this trisomic male is crossed with a (ey ey, gw gw) female, what proportion of the progeny will be phenotypically eyeless? Express your answer as a fraction.

Explanation / Answer

The male will produce the following gametes:

The gametes produced by the female are:

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The cross would give the following progeny:

The progeny which are eyeless should have ey-ey-.

The fraction of the progeny which are eyeless are: 1/2 or 50%

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