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. At the same time, a second gene, N, determines whether or not cacti have spines. Homozygous recessive nn cacti have no spines at all. If cacti heterozygous for both traits, SsNn, were allowed to self-pollinate, the offspring would segregate into which of the following phenotype 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) 9 sharp-spined:3 dull-spined:4 spineless

Question

Quizplus · Accepted Answer

The SsNn x SsNn cross results in a 9:3:3:1 phenotypic ratio, but since nn results in no spines, the 3 sharp-spined:1 dull-spined becomes 9 sharp-spined:3 dull-spined:4 spineless.

Gene S Controls the Sharpness of Spines in a Type