Myotonic dystrophy is an autosomal dominant trait. In a family, an affected male marries a normal female. They produce a normal daughter and an affected son. If this affected son marries a normal female, what is the probability that their first child will be an affected son?

Step-by-step solution

Myotonic dystrophy is an autosomal dominant disorder. Let the dominant allele responsible for the disease be A and the recessive normal allele be a . The normal female in generation I must have the genotype aa . Since the affected male in generation I has a normal daughter ( aa ), he must be heterozygous ( Aa ). The affected son in generation II must also be heterozygous ( Aa ) because he inherits the normal allele ( a ) from his mother. When this affected son ( Aa ) marries a normal female ( aa ), the cross is Aa