Human Heredity 11th Edition by Michael Cummings

Exercise 20

The parents of a 1-year-old boy are concerned that their son may be susceptible to retinoblastoma because the father's brother had bilateral retinoblastoma. Both parents are normal (no retinoblastoma). They have their son tested for the RB gene and find that he has inherited a mutant allele. The father is tested and is found to carry a mutant allele.
a. Why didn't the father develop retinoblastoma?
b. What is the chance that the couple will have another child carrying the mutant allele?
c. Is there a benefit to knowing that their son may develop retinoblastoma?