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 cactuses have spines. Homozygous recessive nn cactuses have no spines at all. Which of the following explain why a crossing a true breeding spineless cactus with a dull-spined cactus could result in both sharp- and dull-spined offspring?
A) Gene S genotype is pleiotropic
B) Sharp-spined cacti are more successful in environments with predators
C) The spineless cactus was heterozygous at the S locus
D) Gene N genotype is polygenic

Question

Quizplus · Accepted Answer

The spineless cactus must carry at least one dominant allele for gene S (for spine sharpness) but is homozygous recessive for gene N (which determines the presence of spines). When crossed with a dull-spined cactus (homozygous recessive at the S locus), the offspring can inherit the dominant S allele from the spineless parent and the recessive n allele from both parents, resulting in cacti with spines (due to the S allele) that can be either sharp or dull depending on the combination of alleles they inherit at the S locus.

Gene S Controls the Sharpness of Spines in a Type