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

• A. All daughters affected, no sons affected
• B. All daughters affected, half of sons affected
• C. Half of daughters affected, half of sons affected
• D. All sons affected, no daughters affected

Answer: (A)

In X-linked dominant inheritance, a single mutant allele on the X chromosome is enough to cause disease. The father contributes his X only to daughters (and his Y to sons). If the father is affected and the mother is unaffected, every daughter inherits the father's mutant X and is affected, while every son inherits a normal X from the mother and is unaffected. Therefore the expected distribution is all daughters affected and no sons affected.