The major types of hemophilia A (also known as classic hemophilia or factor VIII deficiency) and hemophilia B (also known as Christmas disease or factor IX deficiency).

There is 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 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 trauma. This form of the disease, however, is much less predictable than the other types of hemophilia, making it harder to manage.

Inheritance patterns in hemophilia A, B, and B Leyden

Both hemophilia A and B are inherited in an X-linked recessive 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 which could contain either an X or a 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 our bodies.

Because hemophilia is a recessive condition, it only develops if all X chromosomes carry the defective gene. This means that, in men, all it takes is one altered copy of the mutated gene to cause the condition (because men have only one X chromosome, passed on by their mothers).

This is why it is rarer to find women with hemophilia, since they would have to have both X chromosomes containing mutated copies of the gene to have the disease manifest.

A woman with one altered copy of the gene is called a carrier. These women often have the usual amount of coagulation factor (VIII or IX) in their blood, which is necessary for normal blood clotting. However, about 10% of female carriers have less than half the normal amount of coagulation factors and are at higher risks of having abnormal bleeding episodes, particularly after an injury, surgery or tooth extraction.

Inheritance pattern of hemophilia C

Hemophilia C also is primarily inherited, but it does not follow an X-linked recessive pattern. Instead, the gene that causes factor XI deficiency is found on chromosome 4 and 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 risk that the child will inherit one faulty copy of the gene from each parent and develop the condition.

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