Hemophilia: X-Linked Inheritance and Male Predominance Explained

Genetic Basis of Hemophilia

Hemophilia is an inherited bleeding disorder caused by deficiency of clotting factor VIII (hemophilia A) or IX (hemophilia B). The genes for these factors (F8 and F9) are located on the X chromosome.

Why Males Are Predominantly Affected

• Males have one X and one Y chromosome (XY). A single pathogenic variant on their sole X chromosome causes disease.
• Females have two X chromosomes (XX). A variant on one X is usually compensated by the normal allele on the other X, making them asymptomatic carriers (though some have mild symptoms due to skewed X-inactivation).

Inheritance Patterns

• Affected father → all daughters are carriers; no affected sons.
• Carrier mother → 50% chance each son is affected; 50% chance each daughter is a carrier.

Prevalence

• Hemophilia A: ~1 in 5,000–10,000 males
• Hemophilia B: ~1 in 20,000–30,000 males

Clinical Implications

• Genetic counseling for families with known mutations.
• Prenatal testing (CVS, amniocentesis) or preimplantation genetic diagnosis available.
• Carrier females should have factor levels checked before surgery/pregnancy.