Huntington disease is a rare condition in humans, caused by a dominant allele of a single gene, and that results in a slow but inexorable deterioration of the nervous system. the probability that a person with the huntington genotype will express the phenotype varies with age. assume that 50% of those inheriting the hd allele will express the symptoms by age 40. janelle is a 35-year-old woman whose father has huntington disease. she currently shows no symptoms. what is the probability that janelle will show symptoms in five years?