Hemophilia is caused by a deficiency of one of the clotting factors involved in the blood clotting process, most commonly factor VIII or IX. The production of these factors is controlled by a gene; if the gene has a defect it can lead to not enough of the factor being produced. Hemophilia can be inherited because the defective gene can be passed on from parent to child. About 70% of cases of hemophilia are inherited.

Everyone has two sex chromosomes - they inherit one from each parent. Females inherit an X chromosome from their mother and an X chromosome from their father. Males inherit an X chromosome from their mother and a Y chromosome from their father.

The gene that controls the production of factor VIII and IX is located on the X chromosome. As a man only has one X chromosome, if that chromosome carries the defective gene they will have hemophilia. Men can't pass - on the gene that causes hemophilia to their sons. Hemophilia is passed from mother to son via the X chromosome.

Most women who inherit an X chromosome with the defective gene will have a normal version of the gene on their second X chromosome - only if both their inherited X chromosomes have the defective gene will they have hemophilia. Women who have the defective gene on only one of their X chromosomes normally develop no symptoms of hemophilia and are known as carriers of the condition. Women who are carriers have a 50% chance that their sons will have hemophilia.

In about 30% of cases there is no family history of hemophilia. These cases are the result of the gene responsible for producing factor VIII or IX mutating spontaneously, either in a man's sperm or a woman's egg. A mutated gene is often defective, so if a sperm or egg with the mutated gene is fertilized, the child may have hemophilia.