In X-linked dominant inheritance, if the father is affected and the mother is unaffected, what is the expected distribution among sons and daughters?

• A. All daughters affected, all sons affected
• B. All daughters affected, no sons affected (assuming full penetrance)
• C. All sons affected, no daughters affected
• D. Half of sons affected, half of daughters affected

Answer: (B)

In X-linked dominant inheritance, a single mutated copy on the X chromosome can cause the trait, and transmission depends on the parent's sex. If the father is affected and the mother is unaffected, every daughter inherits the father's mutated X while sons inherit the father's Y. The mother contributes a normal X to both sons and daughters. So daughters become X^D X^A and are affected, while sons become X^A Y and are unaffected. This holds best when penetrance is complete; with incomplete penetrance some daughters might not show the trait despite carrying the mutation.