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

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The relationship between genes S and N is an example of epistasis, where the expression of one gene suppresses the expression of another gene. In this case, the presence of the dominant allele S is dependent on the genotype of gene N. If an organism is homozygous recessive for gene N, it will not have any spines, regardless of its genotype for gene S.

Gene S Controls the Sharpness of Spines in a Type