Hemophilia is a blood disorder that is usually inherited. The blood of people with hemophilia does not clot well because they lack a clotting factor.
There are two main types of hemophilia: hemophilia type A (also known as classic hemophilia or factor VIII deficiency) and hemophilia type B (also known as Christmas disease or factor IX deficiency).
There is also a lesser-known form of hemophilia B, known as hemophilia B Leyden, in which patients experience episodes of excessive bleeding in childhood, but have few bleeding problems after puberty.
Hemophilia type C, the rarest type of hemophilia, is a deficiency of blood-clotting factor XI and is characterized by bleeding episodes after dental extractions or similar events.
Acquired hemophilia is a very rare type of hemophilia caused by the immune system mistakenly attacking blood factors that control clotting. This type of hemophilia is not usually inherited, but occurs spontaneously in adults for unknown reasons.
Inheritance patterns of hemophilia A, B, and B Leyden
Both hemophilia A and B are inherited in an X-linked pattern. This is because the genes responsible for the development of these forms of hemophilia are located on the X chromosome.
Humans inherit two sex chromosomes. The mother, who has two X chromosomes, produces an egg containing one X chromosome. The father, who has one X and one Y chromosome, produces sperm cells, which contain either an X or Y chromosome. If the father contributes his X chromosome, a girl is conceived. If he contributes a Y chromosome, a boy is conceived. The sex chromosomes are one of the 23 pairs of human chromosomes that exist in each cell of the body.
If a male child inherits an X chromosome from his mother carrying a disease-causing gene, he will develop the disease because he has only one X chromosome.
Because women have two copies of the X chromosome, it is less likely for them to develop the disease unless they inherit two copies of the disease-causing mutation (one from each parent). This is why hemophilia is rarer in women.
However, the situation is sometimes more complex. Although females have two copies of the X chromosome, one of the two chromosomes is usually inactivated during development to control the number of active genes carried on the X chromosome. Depending on which X chromosome is inactivated, a female may sometimes develop the disease even with a single copy of the disease-causing mutation.
A woman with one altered copy of the gene is called a carrier. These women often have the usual amount of coagulation factor in their blood, which is necessary for normal blood clotting. However, a small percentage of female carriers have less than half the normal amount of coagulation factor and are at higher risk of having abnormal bleeding episodes, particularly after an injury, surgery, or tooth extraction.
Inheritance pattern of hemophilia C
Hemophilia C is also primarily inherited, but it does not follow an X-linked pattern because the mutation that causes it affects a gene found on chromosome 4 (an autosomal or non-sex chromosome). Hemophilia C, therefore, affects both genders equally.
People who carry one faulty copy of the gene causing hemophilia C usually are asymptomatic and do not know they are carriers. If two carriers have a child together, there is a 25 percent chance that the child will inherit one faulty copy of the gene from each parent and develop the disease. There is also a 50 percent chance that any child will be a carrier like the parents, and a 25 percent chance that the child will not inherit a copy of the disease-causing mutation and neither develop the disease nor be a carrier.
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