Gene S controls the sharpness of spines in a type of cactus. Cacti with the dominant allele, S, have sharp spines, whereas homozygous recessive ss cacti have dull spines. Also, a second gene, N, determines whether or not cactuses have spines. Homozygous recessive nn cactuses have no spines at all. What phenotypes would be expected from a cross between a true-breeding sharp-spined cactus and a spineless cactus?
A) all sharp-spined progeny
B) 50% sharp-spined, 50% dull-spined progeny
C) 25% sharp-spined, 50% dull-spined, 25% spineless progeny
D) It is impossible to determine the phenotypes of the progeny.

Question

Quizplus · Accepted Answer

The true-breeding sharp-spined cactus would have the genotype SSNN (since it is true-breeding for sharp spines and has spines), and the spineless cactus would have the genotype ssnn (since it lacks spines due to the nn genotype, the S gene's effect is irrelevant). Crossing these would result in all offspring being SsNn, which have sharp spines because they have at least one dominant allele for sharpness (S) and at least one dominant allele for having spines (N).

Gene S Controls the Sharpness of Spines in a Type