CP Biology – Inheritance Practice Test

What is the genotype notation for a male affected by an X-linked recessive trait?

X^A Y X^a Y X^a X^a X Y

Correct Answer:

X^a Y

Explanation:

In X-linked recessive traits, males are hemizygous for the X chromosome, so the trait shows if the single X they inherit carries the recessive allele. Since the Y cannot carry a corresponding allele to mask it, the recessive allele on the X is enough to express the trait in the male. Therefore, an affected male’s genotype is X^a Y, with X^a representing the X chromosome that carries the recessive allele. If the male were unaffected, he would be X^A Y, where X^A has the dominant normal allele. For females, two copies are needed to be affected (X^a X^a), while a female carrier would be X^A X^a.

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