SAT Subject Biology Practice Question 119: Answer and Explanation

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Question: 119

9. In a system where only two genes are responsible for determining skin pigmentation, a woman is heterozygous for one gene and homozygous for the other. The woman herself only exhibits the homozygous trait. This pattern of expression is known as

A. epistasis
B. X-linkage
C. pleiotropy
D. codominance
E. simple dominance

Correct Answer: A


When more than one gene affects a trait but only one is expressed, this is called epistasis because one gene "stands over" another.

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