Gene s controls the sharpness of spines in a type of cactus. cactuses with the dominant allele. s, have sharp spines, whereas homozygous recessive ss cactuses have dull spines. at the same time, a second gene, n, determines whether or not cactuses have spines. homozygous recessive nn cactuses have no spines at all the relationship between genes s and n is an example of