Huntington's disease is a fatal disorder characterized by a progressive deterioration of the nervous system. The symptoms of this disease usually begin to develop in middle age. It is caused by a dominant allele (H). A man heterogeneous for the Huntington's allele marries a woman who has the homologous recessive phenotype. They plan to have children. What is the probability that they will have a child who develops Huntington's disease in later life?