Which type of inheritance does Hemophilia B fall into?

• A. X-linked dominant
• B. X-linked recessive
• C. Autosomal dominant
• D. Autosomal recessive

Answer: (B)

Hemophilia B is classified as an X-linked recessive disorder. This means that the gene responsible for Hemophilia B is located on the X chromosome, and the condition manifests primarily in males, who have only one X chromosome. In males, one defective copy of the gene is enough for the condition to develop because they do not have a second X chromosome to mask the effect of the mutation.

In contrast, females have two X chromosomes, so they can be carriers of the mutation without showing symptoms if they have at least one normal copy of the gene. Therefore, even if a female inherits one mutated allele, the other normal allele on her other X chromosome can compensate, typically resulting in no symptoms. If a female is homozygous for the mutation, however, she would express the disorder.

This mode of inheritance is characteristic of several genetic conditions linked to the X chromosome, and Hemophilia B specifically occurs due to mutations in the gene encoding factor IX, which is essential for blood clotting. Understanding the X-linked recessive inheritance pattern is crucial for genetic counseling and understanding the probabilities of passing the mutation to offspring.