Rosh Internal Medicine Boost Practice Exam

If a patient has a family history of Huntington disease, what is the probability that she inherited the abnormal gene from her affected father?

1. 100%

2. 25%

3. 50%

4. The patient cannot receive the abnormal gene from her father

The correct answer is: 50%

In the case of Huntington disease, which is an autosomal dominant disorder, an individual only needs one copy of the mutated gene from an affected parent to express the disease. If the father is affected by Huntington disease, he has one abnormal allele and one normal allele. Consequently, when he has children, there is a 50% chance that he will pass on the abnormal allele to each child. Since the question specifically asks about the probability of the patient inheriting the abnormal gene from her affected father, the correct answer reflects the fact that for every child of an affected parent, there is indeed a 50% chance of inheriting the gene that causes the condition. Therefore, the probability that this patient inherited the abnormal gene from her father is accurately represented as 50%.