A Certain Disease Is Caused by Homozygosity of the G

A certain disease is caused by homozygosity of the g allele (G is the corresponding wild-type allele) .However, the penetrance of the disease is 75%.Two individuals known to be heterozygotes have a child.What is the probability that the child exhibits the disease?

A) 1/4
B) 3/4
C) 1/8
D) 3/16
E) 9/16
The child will exhibit the phenotype only if its genotype is gg.And if its genotype is gg, a 75% penetrance means that there is only a 75% chance that the gg genotype will cause the child to exhibit the phenotype.The chance that two heterozygotes (Gg * Gg) have a gg child is 1/4.Multiplying this probability by 75% gives 1/4 * 3/4 = 3/16