In Drosophila, singed bristles (sn) and cut wings (ct) are both caused by recessive, X-linked alleles.The wild type alleles (sn⁺ and ct⁺) are responsible for straight bristles and intact wings, respectively.A female homozygous for sn and ct⁺ is crossed to a sn⁺ct male.The F₁ flies are interbred.The F₂ males are distributed as follows Of these 4 phenotypic classes of offspring, which of the following arose from a parental gamete produced by the F1 females? A)sn⁺ ct⁺ and sn⁺ ct flies B)sn⁺ ct⁺ flies only C)sn ct and sn⁺ ct⁺ flies D)sn⁺ ct and sn ct⁺ flies E)sn ct and sn ct⁺ flies

Question

Quizplus · Accepted Answer

The female parent is sn sn ct+ ct+. The male parent is sn+ sn+ ct ct. The F1 offspring will be all sn sn+ ct+ ct because of the reciprocal crosses. When these flies are intercrossed, we can set up a Punnett square to determine the offspring. Possible gametes from the female: sn ct+: sn ct: sn+ ct+: sn+ ct Possible gametes from the male: sn+ ct: sn+ ct+: sn ct: sn ct+ Crossing these gametes gives the following offspring: sn sn+ ct ct: 1/4 (from sn ct x sn+ ct) sn+ sn ct ct: 1/4 (from sn+ ct x sn ct+) sn sn ct ct+: 1/4 (from sn ct x sn ct+) sn+ sn+ ct+ ct+: 1/4 (from sn+ ct+ x sn ct+) So, the offspring include sn⁺ ct and sn ct⁺ flies (choice D).

In Drosophila, Singed Bristles (Sn) and Cut Wings (Ct) Are