A mother is a carrier of Duchenne muscular dystrophy; the father is unaffected. They have one son with muscular dystrophy. Another male child is expected. The probability of the second son having muscular dystrophy is:

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Duchenne muscular dystrophy is an X-linked recessive disorder. The mother, being a carrier, has a 50% chance of passing the affected X chromosome to her son, resulting in a 50% probability that the second son will have the disorder.

A Mother Is a Carrier of Duchenne Muscular Dystrophy; the Father