Hemophilia runs in families. In the majority of cases, the bleeding disorder is inherited from a parent to a child. But the genetics of hemophilia can be confusing to non-scientists, especially if it’s been a while since you studied biology in high school. Here’s a rundown on the basics of hemophilia inheritance.
How Is Hemophilia Inherited?
“Hemophilia is caused by an alteration in one of the genes (either factor VIII or factor IX) that tells the body to make one of the clotting factor proteins that is needed to form a blood clot,” says Meadow Heiman, MS, CGC, CCRC, a genetic counselor at the Indiana Hemophilia and Thrombosis Center in Indianapolis. “This alteration prevents the body from producing the clotting factor properly.”
People inherit their chromosomes, which contain their genes, from their parents. Chromosomes come in pairs, and humans have 46 chromosomes in 23 pairs. The chromosomes that form the 23rd pair are called the sex chromosomes, and these decide whether a person is born male (XY) or female (XX).
The genes that cause hemophilia are located on the X chromosome. Females have two X chromosomes (one from their mother and one from their father), and males have an X chromosome (from their mother) and a Y chromosome (from their father).
What Gene Causes Hemophilia?
Because males have only one X chromosome, if they inherit the X chromosome that has the altered gene, then their body is unable to produce the clotting factor VIII or IX properly and they have hemophilia.
Because females have two X chromosomes, there are three ways that hemophilia can affect them:
- If only one X chromosome is affected and no bleeding symptoms are present, then these females are said to be “carriers” of hemophilia. In other words, they carry the gene that causes hemophilia on the X chromosome and can pass that affected gene on to their children.
- If only one X chromosome is affected but bleeding symptoms are present and they have lower than normal factor levels, then these females have hemophilia.
- In the rare instances in which both copies of a female’s factor VIII or IX gene are affected, these females have hemophilia.
Hemophilia Inheritance Patterns
There are three possible scenarios for inheriting hemophilia, depending on whether a mother or father (or both) is affected:
If the mother has one altered factor gene and the father does not have hemophilia, each son has a 50% chance of having hemophilia and each daughter has a 50% chance of having one altered factor gene, causing her to be a carrier or have hemophilia.
If the father has hemophilia, and the mother does not carry a hemophilia gene, then each son has a 0% chance of having hemophilia, and each daughter has a 100% chance of either being a carrier or having hemophilia.
If the father has hemophilia, and the mother is a carrier of the hemophilia gene, then each son has a 50% chance of having hemophilia, and each daughter has a 50% chance of having one altered factor gene causing her to be a carrier or have hemophilia, and a 50% chance of having two altered factor genes and having hemophilia.
When There’s No Family History
About one-third of people who are diagnosed with hemophilia have no family history of the bleeding disorder. This happens when a new alteration (mutation or change) occurs in the factor VIII or IX gene. New changes in our genes normally happen when we pass our genetic material from parent to child. If a new change by chance occurs in the factor VIII or IX gene, then it introduces hemophilia into a family.
“Once a person has this alteration in their hemophilia gene, they will be able to pass it on to future generations,” says Heiman.