X-linked recessive traits are more common in males. How can a carrier mother affect offspring?

• A. Males require two copies of the mutant X to express the trait.
• B. X-linked recessive traits cannot be passed from mothers to offspring.
• C. Males inherit the X from their father, not their mother.
• D. Carrier mothers can pass the mutant X to half of their sons and half of their daughters; daughters may be carriers or affected depending on the father.

Answer: (D)

This question is about how X-linked recessive traits are passed from a carrier mother to her children. In people, males have one X and one Y, while females have two Xs. An X-linked recessive trait shows up in males if they inherit a single mutant X, because they don’t have a second X to mask it. A carrier mother has one normal X and one mutant X, so each pregnancy gives a 50% chance for the child to inherit the mutant X.

A son receives his X from his mother and his Y from his father, so if the mother passes the mutant X, the son will be affected. That happens about half the time. A daughter receives one X from each parent; if she inherits the mutant X from her mother, she becomes a carrier (assuming the father’s X is normal). If the father is affected (has the mutant X), a daughter could be affected as well, but with a normal father, daughters who get the mutant X from the mother are typically carriers. So overall, a carrier mother can pass the mutant X to half of her sons (affecting them) and half of her daughters (making them carriers), with daughters’ status depending on the father’s genotype.