Hemophilia is almost always inherited and is "X-linked." This means the gene responsible for the condition sits on the X chromosome.

Severity is classified by how much "factor" is present in the blood. Those with hemophilia may experience "spontaneous bleeds"—bleeding that happens for no clear reason, often into the knees, elbows, or ankles. Modern Management

Since females have two X chromosomes (XX), a healthy gene on one can usually compensate for a faulty one on the other. The Royal Connection

Since males have only one X chromosome (XY), a single faulty gene causes the disease.