Final answer:
The son of a woman heterozygous for an X-linked recessive muscular dystrophy and an affected man has a 50% chance of being affected by the disease, because he will inherit the disease if he receives his mother's affected X chromosome.
Step-by-step explanation:
The question concerns X-linked recessive inheritance patterns in relation to a specific type of muscular dystrophy. In cases where a muscular dystrophy is X-linked and the father is affected and the mother is heterozygous for the trait, the inheritance probability for a son must be considered. The mother can pass either her affected X chromosome or her normal X chromosome.
Since males have one X and one Y chromosome, they will be affected if they inherit the affected X chromosome from the mother. The father's Y chromosome does not carry the gene for the disease as X-linked disorders are not transmitted through the Y chromosome. Therefore, the probability that the son will be affected is 50%, as the son will be affected if he inherits the mother's affected X chromosome, but will not be affected if he inherits the mother's unaffected X chromosome.
Daughters, on the other hand, will inherit an X chromosome from the mother and one from the father. Even if a daughter inherits the affected X chromosome from the mother, she will not be affected by the disease if she also inherits her father's normal X chromosome because she will be a carrier like her mother. However, if she were to inherit the affected X chromosome from both parents, which in this scenario is not possible because the father is affected and has no affected X to pass on, then she would be affected by the disease. Therefore, all daughters in this scenario would at least be carriers, with a 0% chance of being affected.