Peter is a 30-year old man whose father died of Huntington's disease. Neither Peter's mother nor a much older sister show any signs of Huntington's. What is the probability that Peter has inherited Huntington's disease?

50%

Explanation:

Huntington's disease is an autosomal dominant disorder that leads to progressive brain impairment and cognitive problems

The probability that Peter can inherit this disease from his father is 50% (who is carrier of the defective allele), having into account that no sign of this disorder is observed from the mother's family