SAT Subject Biology Practice Question 111: Answer and Explanation

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Question: 111

1. If the allele for dark-colored hair is dominant over the allele for light-colored hair, which is LEAST likely?

A. A light-haired couple produces a baby with dark hair.
B. A dark-haired couple produces a baby with light hair.
C. A light-haired woman and a dark-haired man produce a light-haired baby.
D. A dark-haired woman and a light-haired man produce a light-haired baby.
E. A dark-haired woman and a light-haired man produce a dark-haired baby.

Correct Answer: A

Explanation:

Both parents must be homozygous recessive, thus rendering them incapable of producing a dark-haired baby displaying the dominant trait. In Choice B, if both parents are heterozygous, they can display dark hair but still produce a light-haired baby. In Choice C we know that the mother is homozygous recessive, but we do not know whether the paternal genotype is homozygous dominant or heterozygous. If the father is heterozygous, it is possible that the couple could produce a light-haired child, making this an incorrect answer. Choice D is the same scenario as Choice C, but with the parents' genotypes reversed. In Choice E, no matter the woman's genotype, the couple may produce a dark-haired baby.

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