Duchenne muscular dystrophy is a recessive disorder caused by a rare, loss-of-function allele that is located on the X chromosome in humans. An unaffected woman (i.e., without disease symptoms) who is heterozygous for the X-linked allele causing Duchenne muscular dystrophy has children with a man with a functional (non-diseasecausing) allele. What is the probability that this couple will have an unaffected son?