Question

At age 46, Sam begins to slur his speech and stagger when he walks. People think he is intoxicated, but        Sam does not drink alcohol. His children urge him to seek medical care; he learns he has Huntington     disease (HD), an adult-onset, autosomal dominant condition. One of his sisters, Pam, undergoes genetic        counseling and testing, and is found to be free of the mutation. She takes care of Sam, as she took    care of him as a child after their parents died in a car crash in their thirties. Another sister, Sue, refuses    counseling and testing.

                                   

            a)         Draw a pedigree for this family.

            b)         What is the risk that Sam’s daughter has inherited HD?            ___________

            c)         What is the risk that Sue’s son has inherited HD?         ___________

            d)         When Sue learns Pam is free of the mutation, she assumes her own risk is                                            increased. Is she correct? ___________ Explain your answer.

Answer 1

Huntington disorder is an autosomal dominant disease.
Dominant mutant allele = D
WT recessive allele = d

See the image for the pedigree
Sam's mother or father or both have to be affected.
At least one of his parents is affected.
In the given pedigree, it is shown that his mother is affected.

His mother His father bo Sue Sam Dd Pam dd

b. Sam = Dd
The probability that his daughter inherits the mutant allele = 1/2 = 50%

c. Sue = dd
The probability that her son inherits the mutant allele = 0%

d. No
The birth of each individual is a seperate event and the probability for any of them to acquire mutant allele is 50%.
It does not increase or decrease with the number of progeny.