Jeff's father has Huntington disease, which is caused by a rare autosomal dominant allele (H ). Two-thirds of people with the dominant Huntington allele show no symptoms by age 50, and Jeff is 50 and has no symptoms. Jeff's wife is is pregnant. What is that chance that Jeff's child will eventually show symptoms of Huntington disease?

a. 1/8
b. 1/4
c. 1/5
d. 1/6
e. 1/12
f. 1/24
g. 2/3

Which one?