Question

5. Bill has come to you for genetic counseling. He is concerned that he and his sister (Alice) are at risk for Huntington disease. Bill and Alice are too young to show symptoms. Their grandmother, (Mary) had Huntington disease. Her three children, Shirley, Tom, and Ed so far show no symptoms. Shirley is the oldest (45 years old) and is Bill and Alice’s mother. A study by Adams (American Journal of Human Genetics 43;695 – 704, 1988) indicates that 68% of individuals heterozygous for the Huntington allele show symptoms by age 45.

a. Draw a pedigree of this family.

b. What is the probability that Bill is heterozygous for Huntington disease?

c. What test(s) would you recommend for Bill and Alice to determine if they are heterozygous?

d. Explain why you recommend this test, what results it can give, and how these results can answer Bill’s question. PLEASE BE SPECIFIC

Answer 1

Huntington is an autosomal dominant disorder, so every body who have one affected parent has 50% chance to have the disease, if other parent is homozygous normal.

The answers of qustions a and b is shown below:

c. I will recommend both neurological and genetic test to test whether they are heterozygous or not. Genetic test includes counting of CAG repeats present in the HD gene from both the number 4 chromosome in which the HD gene is located.

d. It is found that the persons who develop HD at the later stage have CAG repeats more than 30 times in their HD gene , located on 4th Chromosome. So, if the number of CAG repeat is more than 30 in bill's HD gene, then we can predict, bill has a greater risk of developing HD when he will be 45 or more.