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Grandchildless

1. You have isolated a recessive Drosophila mutant that you call grandchildless (gls) because it behaves in the following way:

Cross: gls/gls female x +/+ male
F1: All sterile

If you perform the reciprocal cross, however, you find that the result is different from that in cross 1:

Cross: gls/gls male x +/+ female
F1: All fertile
F2 (from crossing F1 males and females): All fertile
F3 (from crossing F2 males and females): ¼ sterile, ¾ fertile

a. Explain the grandchildless mutant. If the fertile F3 are mated, what will the ratio of fertile:sterile be in the F4?
b. You isolate a new allele of the gls gene (gls*) which behaves in the following way:
Cross: gls/gls female x gls*/+ male
F1: ½ fertile, ½ sterile
Propose an explanation for the new allele of gls.

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