If a woman with a disease trait, which is X-linked recessive, has children with an unaffected male, which of the following statements would be most accurate?

Question

BlackTom AI · Accepted Answer

For X-linked recessive traits, a female who carries one mutant X allele (heterozygote, Xr X+) can pass either the mutant Xr or the normal X+ to each child with equal probability. Males are XY, so a son who receives Xr (from his mother) will be affected (Xr Y), whereas a daughter needs two mutant Xs (Xr Xr) to be affected; a daughter who receives one mutant X will be a carrier (Xr X+) but usually not affected.

Step-by-step reasoning with a Punnett-square style breakdown:
- Parental genotypes (typical interpretation of the question is that the woman is a carrier): Mother = Xr X+; Father = X+ Y.
- Maternal gametes: Xr or X+ (50% each). Paternal gametes: X+ or Y (50% each).
- Possible offspring genotypes (each 25% of all conceptions):
  1) X+ (mother) + X+ (father) → X+ X+ (daughter, unaffected, not carrier)
  2) Xr (mother) + X+ (father) → Xr X+ (daughter, carrier, usually unaffected)
  3) X+ (mother) + Y (father) → X+ Y (son, unaffected)
  4) Xr (mother) + Y (father) → Xr Y (son, affected)

From this:
- 50% of daughters will be carriers (Xr X+) but essentially 0% of daughters will be affected in this heterozygous-carrier scenario, so option (a) is incorrect.
- 25% of all children (one of the four boxes) will be affected, so option (b) (“50% of her children”) is incorrect.
- Of the sons specifically, two possible outcomes are X+Y (unaffected) or XrY (affected) — each with 50% probability. Therefore 50% of her sons are expected to have the trait (option c is correct).
- Option (d) (“All of her children will have the trait”) is incorrect unless the mother were homozygous Xr Xr (rare); that is not the usual carrier scenario implied.

Thus the correct choice is c. 50% of her sons would be expected to have the trait.

If a woman with a disease trait, which is X-linked recessive, has children with an unaffected male, which of the following statements would be most accurate?单项选择题

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