In an X-linked recessive cross between an affected male (X^aY) and an unaffected female (X^AX^A), which outcome best describes the offspring?

In an X-linked recessive cross between an affected male (X^aY) and an unaffected female (X^AX^A), which outcome best describes the offspring?

• A. All daughters are carriers (X^AX^a) and all sons are unaffected (X^AY).
• B. All daughters are affected and all sons are carriers.
• C. Daughters are affected and sons are unaffected.
• D. Daughters are unaffected and sons are carriers.

Answer: (A)

This question tests how X-linked recessive traits are passed from a father to his children. In X-linked recessive inheritance, males have only one X chromosome, so a single recessive allele on that X will express the trait, while females need two copies of the recessive allele to be affected. Crossing an affected male (X^a Y) with an unaffected female (X^A X^A) gives offspring where the daughters receive the X^a from the father and X^A from the mother, yielding X^A X^a. That makes them carriers (one normal X and one mutant X) but not affected. The sons receive the Y from the father and X^A from the mother, yielding X^A Y, so they are unaffected. So the offspring are: daughters who are carriers and sons who are unaffected. This matches the described outcome.

This question tests how X-linked recessive traits are passed from a father to his children. In X-linked recessive inheritance, males have only one X chromosome, so a single recessive allele on that X will express the trait, while females need two copies of the recessive allele to be affected.

Crossing an affected male (X^a Y) with an unaffected female (X^A X^A) gives offspring where the daughters receive the X^a from the father and X^A from the mother, yielding X^A X^a. That makes them carriers (one normal X and one mutant X) but not affected. The sons receive the Y from the father and X^A from the mother, yielding X^A Y, so they are unaffected.

So the offspring are: daughters who are carriers and sons who are unaffected. This matches the described outcome.