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

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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) oci are tightly linked on chromosome IV. Loss-of-function ey and gu alleles are recessive to ey and gw respectively. Flies homozygous for ey lack eyes and flies homozygous for gu ave 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: ey gW eyt gw For the progeny of this cross to be wild-type, they need to receive both the (ey gm and ey chromosomes from the trisomic male. This combination is present in 1/6 of the trisomic male's sperm. Part D If this trisomic male is crossed with a (ey ey gwT gw female, what proportion of the progeny will be phenotypically eyeless? Express your answer as a fraction (example: 3/5). Submit My Answers Give Up Part E This question will be shown after you complete previous question(s). Continue Provide Feedback

Explanation / Answer

Cross is: ey-gw-/ey+gw-/ey-gw+ crossed to ey-gw-/ey-gw-

Gametes of female will be ey-gw-

For males: It is given that 1/6th of the total males are wild type with the combination ey+gw-/ ey-gw+

½ of this 1/6th can be eyeless because female gamete will always be “ey-”. So, eyeless will be ½*1/6 = 1/12

Rest 5/6 males have the combination either ey-gw-/ey+gw- or ey-gw-/ey-gw+. Here, it should be given that how many males are having ey-gw-/ey+gw- and how many are having ey-gw-/ey-gw+.

Let us consider that half progeny is ey-gw-/ey-gw+ and the rest half is ey-gw-/ey+gw-.

½ of 5/6 will be eyeless if the males are ey-gw-/ey+gw- = 5/12 *1/2 = 5/24

Whole of 5/6 will by eyeless if males are ey-gw-/ey-gw+ = 1*1/2 = ½

Total eyeless progeny out of the rest of the (5/6) males: (1/2) + (5/24) = (12+5)/24 =17/24

Total eyeless progeny out of 1/6 and 5/6 = (1/12) + (17/24) = 19/24

** Hope it helps. Please note that it is the answer only if half progeny is ey-gw-/ey-gw+ and the rest half is ey-gw-/ey+gw-.

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