The disorder is caused by an inherited sex-linked recessive trait with the defective gene located on the X chromosome. Females carry two copies of the X chromosome, and therefore if the factor VIII gene on one chromosome is defective, the gene on the other chromosome can compensate. Males, however, carry only one X chromosome, so if the factor VIII gene on that chromosome is defective, they will have the disease. 

Females with one defective factor VIII gene are carriers of this trait. Fifty percent of the male offspring of female carriers have the disease and 50% of their female offspring are carriers. All female children of a male hemophiliac are carriers of the trait.

The severity of symptoms can vary with this disease, and the severe forms become apparent early on. Bleeding is the hallmark of the disease and sometimes, though not always, occurs if an infant is circumcised. Additional bleeding manifestations make their appearance when the infant becomes mobile.

Mild cases may go unnoticed until later in life when they occur in response to surgery or trauma. Internal bleeding may happen anywhere, and bleeding into joints is common. Risk factors are a family history of bleeding and being male.

The incidence of hemophilia A is 1 out of 5,000 men.

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