Huntington's disease is an autosomal dominant disorder that affects the nervous system. if a man is heterozygous for the alleles that cause the condition and has children with a woman who lacks that defective allele, which of these statements is true?

Since Huntington's disease is an autosomal dominant trait, then even in the presence of only one affected allele, the person will manifest the disease. The man in the question has Huntington's disease. If he will have children with his wife who has no gene for Huntington's disease, then there will be a 50% chance that his children will develop Huntington's disease. 

Attached is a punnet square concerning the genes responsible for Huntington's disease.