SAT Subject Biology Practice Question 25: Answer and Explanation

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Question: 25

2. A woman expresses an X-linked recessive disorder. Which of these conclusions can DEFINITELY be made about the woman's parents?

I. Her mother passed on an allele for the disorder.

II. Her father expresses the same disorder.

III. Her mother does not express the disorder.

A. I only
B. II only
C. I and II only
D. II and III only
E. I, II, and II

Correct Answer: C


The woman inherited two X chromosomes, one from her father and one from her mother. Because the disorder is recessive, both of her X chromosomes must hold the allele for the disorder. Because one X chromosome must have come from the father (who has only one X), he must also express the disorder. Because the second X chromosome must have come from the mother, she definitely passed on the allele. Whether the mother also expresses the disorder depends on the allele on her other X chromosome.

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