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 cacti have spines. Homozygous recessive nn cacti have no spines at all. The relationship between genes S and N is an example of which of the following inheritance patterns?

Question

epistasis
incomplete dominance
pleiotropy
codominance

Answer 1

The relationship between genes S and N in your description is an example of epistasis. In this case, the presence or absence of spines is determined by the N gene, while the sharpness of the spines is determined by the S gene. If the cacti are homozygous recessive for the N gene (nn), they will not have spines at all, regardless of the genotype at the S gene. Thus, the N gene can mask the expression of the S gene, which is characteristic of an epistatic interaction.