Rosh Internal Medicine Boost Practice Exam

What is the mode of genetic transmission for hemophilia B?

1. Autosomal dominant

2. Autosomal recessive

3. X-linked dominant

4. X-linked recessive

The correct answer is: X-linked recessive

Hemophilia B is a genetic disorder that specifically involves a deficiency in factor IX, a clotting factor that is essential for proper blood coagulation. The mode of transmission for hemophilia B is classified as X-linked recessive. This means the gene responsible for hemophilia B is located on the X chromosome. In X-linked recessive disorders, females have two X chromosomes (XX), so they can be carriers if they have one affected X chromosome, but they usually do not exhibit symptoms unless both X chromosomes are affected. Males, on the other hand, have one X and one Y chromosome (XY). Therefore, if a male inherits the affected X chromosome, he will express the disorder because he lacks a second X chromosome that could provide a normal allele. This pattern explains why hemophilia B predominantly affects males, while females can be carriers. In situations where the affected X chromosome is passed from a carrier mother to her sons, there is a 50% chance of them being affected. Conversely, daughters would have a 50% chance of becoming carriers. Understanding this inheritance pattern is crucial for genetic counseling and management of families affected by hemophilia B, particularly for assessing risks for future generations.