Chapter 5, Problem 10P

In a particular human family, John and his mother both have brachydactyly (a rare autosomal dominant allele causing short fingers). John’s father has Huntington disease (another rare autosomal dominant allele). John’s wife is phenotypically normal and is pregnant. Two-thirds of people who inherit the Huntington (HD) allele show symptoms by age 50, and John is 50 and has no symptoms. Brachydactyly is 90% penetrant.

a.	What are the genotypes of John’s parents?
b.	What are the possible genotypes for John? How likely is John to have each of these genotypes?
c.	What is the probability the child will express both brachydactyly and Huntington disease by age 50 if the two genes are unlinked?
d.	How will your answer to part (c) change if instead these two loci are 20 m.u. apart?