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. 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

To determine the phenotype ratio, we use the dihybrid cross method. The genotype of the parent cacti is SsNn. When these cacti self-pollinate, the offspring can have the following genotypes and phenotypes: SSNN, SSNn, SsNN, SsNn (sharp-spined); ssNN, ssNn (dull-spined); and nn (spineless, regardless of the S gene because N is epistatic to S). The phenotypic ratio is 9 sharp-spined:3 dull-spined:4 spineless.

Gene S Controls the Sharpness of Spines in a Type