Hemophilia is a genetic disease passed down through families, not something you catch or develop on your own

Hemophilia is caused by a mutation in a gene that tells your body how to make clotting proteins. You are born with this mutation—it comes from your parents' DNA, not from anything that happens during your life. The mutation is permanent and present in every cell of your body from conception onward.

The two main types, hemophilia A and hemophilia B, follow the same inheritance pattern. Both genes sit on the X chromosome, which is why the disease shows up differently in men and women. A man with the mutation will have hemophilia. A woman with one mutated copy is usually a carrier but may have symptoms depending on which X chromosome her cells use.

Key Takeaways

  • Hemophilia is caused by a mutation in a single gene on the X chromosome that you inherit from your parents before birth.
  • Men who inherit the mutation will have hemophilia; women who inherit it are usually carriers but may have bleeding symptoms.
  • If your mother is a carrier, you have a 50 percent chance of inheriting the mutation; if your father has hemophilia, all daughters will be carriers.
  • About one-third of hemophilia cases occur in people with no family history, meaning the mutation happened for the first time in that person.
  • Genetic testing can confirm whether you carry the mutation and help you understand the risk to your children.

How the X chromosome determines who gets hemophilia

The hemophilia gene sits on the X chromosome. Men have one X chromosome (XY) and women have two (XX). This difference is why hemophilia affects men and women differently.

A man with one mutated X chromosome will have hemophilia because he has no second X chromosome to compensate. A woman needs mutations on both X chromosomes to have hemophilia, which is rare. If a woman has a mutation on only one X chromosome, she is a carrier—she carries the gene but usually does not have severe bleeding. However, some carrier women do experience bleeding symptoms because of how their cells randomly choose which X chromosome to use.

What happens when a carrier mother has children

If your mother is a carrier, she has one normal X chromosome and one mutated X chromosome. When she has children, each child has a 50 percent chance of inheriting the mutated chromosome.

Sons who inherit the mutated X from their mother will have hemophilia. Daughters who inherit it will be carriers like their mother. Sons who inherit the normal X from their mother will not have hemophilia and are not carriers. Daughters who inherit the normal X will not be carriers, but they could still inherit a mutated X from their father if he has hemophilia.

What happens when a father with hemophilia has children

A father with hemophilia has one mutated X chromosome (and one Y chromosome). He passes his X chromosome to all his daughters and his Y chromosome to all his sons.

All daughters of a man with hemophilia will be carriers because they inherit his mutated X. Sons will not have hemophilia and will not be carriers because they inherit his Y chromosome, not his X. However, if the mother is also a carrier or has hemophilia, the sons could inherit a mutated X from her.

New mutations that appear without family history

About one in three people diagnosed with hemophilia has no family history of the disease. This happens when a new mutation occurs in the egg or sperm cell before conception, or very early in fetal development. The mutation was not present in either parent but arose spontaneously.

If you are the first person in your family with hemophilia from a new mutation, your children still face the same inheritance risks as anyone else with hemophilia. Your siblings and their children are not at increased risk unless they also inherited the same mutation, which is extremely unlikely.

Genetic testing to understand your inheritance risk

A blood test can identify the specific mutation causing hemophilia in your family. Genetic testing shows whether you carry the mutation, which helps you understand the risk to your children and can guide decisions about family planning.

If you are a woman and want to know whether you are a carrier, or if you are planning to have children, talk to your doctor about genetic testing or a referral to genetic counseling. A genetic counselor can explain what your test results mean for your family and discuss options. Testing is not urgent and can be done at any point in your life.

Why hemophilia is not contagious or caused by lifestyle

Hemophilia cannot be caught from another person and does not develop because of diet, injury, or anything you do during your life. It is purely genetic—determined by the DNA you inherited. You cannot prevent it, and it is not a sign of anything you or your parents did wrong.

The mutation that causes hemophilia is a change in the DNA sequence. It happens to occur in a gene that controls clotting protein production. Some mutations are harmful, some are neutral, and some are beneficial. The hemophilia mutation is harmful because it disrupts a protein your blood needs to clot normally.

Frequently Asked Questions

If I have hemophilia, will all my children have it?

No. If you are a man with hemophilia, all your daughters will be carriers but will not have hemophilia (unless their mother is also a carrier). All your sons will not have hemophilia or be carriers. If you are a woman with hemophilia, each child has a 50 percent chance of having hemophilia or being a carrier, depending on the father's status.

Can hemophilia skip generations?

Yes. A woman can be a carrier with no symptoms and pass the mutation to her son, who will have hemophilia. Or she can pass it to her daughter, who becomes a carrier. The mutation is present in both generations but may not cause noticeable bleeding in the carrier generation.

What does it mean if I am a carrier?

You carry one copy of the hemophilia mutation but usually do not have the bleeding problems that people with hemophilia experience. However, some carriers do have mild bleeding symptoms. Carriers can pass the mutation to their children. Genetic counseling can help you understand your specific situation.

Can hemophilia be cured by fixing the gene?

Gene therapy is a newer treatment that can reduce or stop bleeding in some people with hemophilia, but it is not widely available yet and does not work the same way for everyone. It is different from a cure because it treats the symptoms rather than permanently rewriting your DNA. Talk to your doctor about whether gene therapy might be an option for you.

If my parents do not have hemophilia, how do I have it?

A new mutation can occur in the egg or sperm that created you, even if neither parent has hemophilia or carries the mutation. This accounts for about one-third of hemophilia cases. Your parents' genes are normal; the mutation happened for the first time in you.