A dominant trait,Huntington disease,causes severe neural/brain damage at approximately age 40.The gene that causes this disease is not located on the sex chromosomes.A female whose mother has Huntington disease marries a male whose parents are normal.It is not known if the female has the disease.Assuming the female's mother was a heterozygote,and her father was normal,what is the probability that their firstborn will inherit the gene that causes Huntington disease?