In cactus, the relationship between Gene S and Gene N is known
to be epistatic. 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, the second gene, N, determines whether or not cacti have
spines. Homozygous recessive nn cacti have no spines at all.
Briefly explain why this genetic interaction is considered
epistasis. Which gene is...