Almost all persons who have hemophilia B are born with it. For information on non-genetic causes of bleeding disorders, please click here. Hemophilia B is not contagious and does not spread like a virus or an infection. Read on for more information about the genetics of hemophilia B and how it is inherited.

The gene that contains the instructions to properly make FIX is on the X chromosome. Hemophilia B is caused when that gene is defective. Since males have one Y and one X chromosome, hemophilia affects mostly males. Women have two X chromosomes, so even if one X chromosome is defective, the other sometimes has the information needed to create clotting factors. A female can have hemophilia B because of X-inactivation or lyonization. Hemophilia may be inherited, or it may be caused by a spontaneous genetic mutation of the factor gene occurring in either the mother or in the child.

Carriers of Hemophilia B

A female who has a defect in one of her X chromosomes is said to be a carrier of hemophilia. She carries the gene that may be passed to her children. A mother may be a carrier and may not know that she is a carrier if there is no family history of the condition.

In 70% of hemophilia cases, there is a known family history. The gene that causes hemophilia is passed from parent to child. A mother that carries the gene is called a carrier, and she has a 50% chance of having a son with hemophilia and a 50% chance of having a daughter who is also a carrier. A father who has hemophilia possesses the gene and passes it on to his daughter because daughters receive two X chromosomes, one from their mother and one from their father. This is why daughters of men with hemophilia are called obligate carriers. Since sons only receive a Y chromosome from their father, they would not have inherited hemophilia. Please view the illustrations demonstrating the hemophilia inheritance patterns to learn more.

Inheritance Pattern of Hemophilia B

The following diagrams show how the hemophilia B gene can be inherited. In approximately one-third of people with hemophilia, there is no family history of the disorder, and the genetic mutation is usually spontaneous.




50% chance that each son will have hemophilia.
50% chance that each daughter will be a carrier of the hemophilia gene.
50% chance that each daughter will have hemophilia.

Spontaneous Mutations

Approximately 30% of cases of hemophilia are caused by a spontaneous mutation of the gene. In some cases, the mother is not a carrier of hemophilia, and the child is affected. This child may be the first in the family to have hemophilia and to carry the defective factor gene. In some cases, the genetic mutation occurs in the mother, and she finds out she is a carrier after she has a child with hemophilia (through DNA testing).

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