Haemophilia B Inheritance Mode Explained

Haemophilia B is a genetic bleeding disorder. Understanding its inheritance pattern is crucial.

Understanding Haemophilia B Genetics

• Haemophilia B results from a deficiency in clotting Factor IX.
• The gene responsible for Factor IX (F9 gene) is located on the X chromosome.
• Genes located on the sex chromosomes determine sex-linked inheritance patterns.

X-linked Recessive Inheritance

The inheritance of haemophilia B follows an X-linked recessive pattern:

• Gene Location: The F9 gene is on the X chromosome (specifically Xq27.1).
• Recessive Nature: An individual must have the mutation on both X chromosomes (if female) or the single X chromosome (if male) to express the disorder.
• Males (XY): Are affected if they inherit the mutated gene on their single X chromosome from their mother.
• Females (XX): Have two X chromosomes. They are typically carriers if they inherit one mutated gene and one normal gene. They are usually only affected if they inherit mutated genes on both X chromosomes, which is rare.

Evaluating Other Inheritance Patterns

• Autosomal Dominant/Recessive: These patterns involve genes on non-sex chromosomes (autosomes). Haemophilia B is not linked to autosomal genes.
• X-linked Dominant: In this pattern, a single mutated gene on the X chromosome is sufficient to cause the disorder in both males and females. This differs from the recessive nature of haemophilia B.

Based on the location of the F9 gene and its recessive effect, haemophilia B is correctly described as X-linked recessive.