Gene S controls the sharpness of spines in a type of cactus. Cactuses with the dominant allele, S, have sharp spines, whereas homozygous recessive ss cactuses have dull spines. At the same time, a second gene, N, determines whether or not cactuses have spines. Homozygous recessive nn cactuses have no spines at all. If doubly heterozygous SsNn cactuses were allowed to self-pollinate, the F2 would segregate in which of the following ratios?
A) 3 sharp-spined:1 spineless
B) 1 sharp-spined:2 dull-spined:1 spineless
C) 1 sharp-spined:1 dull-spined:1 spineless
D) 1 sharp-spined:1 dull-spined
E) 9 sharp-spined:3 dull-spined:4 spineless

Question

Quizplus · Accepted Answer

The correct segregation ratio for the F2 generation from a doubly heterozygous SsNn self-pollination is 9 sharp-spined:3 dull-spined:4 spineless. This follows from applying the dihybrid cross principle, where each gene segregates independently, resulting in a 9:3:3:1 ratio for the phenotypes, with the modification that the nn genotype for the N gene results in spinelessness, overriding the S gene's effect.

Gene S Controls the Sharpness of Spines in a Type