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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
A) incomplete dominance.
B) epistasis.
C) complete dominance.
D) pleiotropy.
E) codominance.

Question

Quizplus · Accepted Answer

The relationship between genes S and N is an example of epistasis, which means that the expression of one gene (S) is dependent on the presence or absence of another gene (N). In this case, the presence of the dominant allele S alone is not enough to produce sharp spines, as the cactus must also have at least one dominant allele of N to have spines at all. This is an example of gene interaction, rather than complete dominance or codominance. Pleiotropy refers to the ability of a single gene to affect multiple traits, while incomplete dominance involves blending of two alleles.

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