Essential Genetics 6th Edition by Daniel Hartl

Exercise 8

Huntington's disease is a rare neurological degenerative disorder resulting from a rare dominant mutant allele designated HD. The allele has complete penetrance, but the disorder is usually manifested after the age of 45. A young man has learned that his father has developed the disease.
(a) What is the probability that the young man will later develop the disorder?
(b) What is the probability that a child of the young man carries the HD allele?