Dextral (D) and sinistral (d) shell coiling in the water snail Limnaea peregra is genetically determined. From a population of mixed sinistral and dextral snails, a sinistral female and a dextral male are chosen. All the offspring are dextral. When the female offspring are mated, 50% produce only sinistral snails and the other 50% produce only dextral snails.
-What were the genotypes of the sinistral female and dextral male that were initially chosen?
A)dd female and Dd male
B)Dd female and dd male
C)dd female and DD male
D)DD female and dd male
E)Dd female and Dd male

Question

Quizplus · Accepted Answer

The fact that all the offspring from the mating of a sinistral female and a dextral male are dextral suggests that the dextral trait is dominant over sinistral trait. This means that the male must be heterozygous for the dextral trait (Dd) while the female could be either homozygous for the sinistral trait (dd) or heterozygous (Dd). If the female was homozygous for the sinistral trait (dd), then all the offspring from the second mating of the female offspring would also be sinistral but this is not what is observed. Therefore, it is likely that the female was heterozygous for the sinistral trait (Dd) and the male was heterozygous for the dextral trait (Dd).

Dextral (D) and Sinistral (D) Shell Coiling in the Water