Hemophilia's Male Predominance: The Role of X-Chromosome Genetics

Hemophilia results from mutations in F8 (hemophilia A) or F9 (hemophilia B), both on the X chromosome.

Why Males Are Primarily Affected

• Males (46,XY): Single X chromosome → no backup copy. One mutation = disease.
• Females (46,XX): Two X chromosomes. Random X-inactivation usually silences the mutant allele in enough cells to maintain adequate factor levels.

Female Manifestations

• True homozygous/compound heterozygous females: severe disease (very rare).
• Symptomatic carriers: skewed X-inactivation → factor levels <40% → mild bleeding phenotype.
• Obligate carriers: all daughters of affected males.

Thus, hemophilia is overwhelmingly a male disease, but females are not immune to bleeding symptoms.