Yes, hemophilia is genetic—it's caused by a mutation in a gene that controls blood clotting
Hemophilia runs in families because it results from a change in the DNA instructions your body uses to make clotting proteins. The two main types—hemophilia A and hemophilia B—each involve a mutation in a different gene. If you have hemophilia, that mutation is in your cells right now. If you're a parent, you may pass it to your children. Understanding how this inheritance works helps you know what to expect for yourself and your family members.
The mutation itself is permanent and present from birth, though symptoms may not appear until later in childhood or even adulthood. You cannot catch hemophilia or develop it later in life from an injury or illness—you either inherit the mutation or you don't. About one-third of people diagnosed with hemophilia have no family history, meaning the mutation arose spontaneously in them rather than being passed down.
Key Takeaways
- Hemophilia A and B are caused by mutations in specific genes on the X chromosome, which is inherited from your parents.
- Males with the mutation almost always have hemophilia symptoms, while females with one mutated copy are usually carriers and may have mild or no symptoms.
- If your mother carries the mutation, you have a 50 percent chance of inheriting it; if your father has hemophilia, all daughters will be carriers and no sons will inherit it.
- About one-third of hemophilia cases result from a new mutation rather than inheritance from a parent.
- Genetic testing can confirm whether you carry the mutation and help predict which family members may be affected.
How the X chromosome determines who gets hemophilia
Both hemophilia A and B are X-linked recessive conditions, meaning the faulty gene sits on the X chromosome. Males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes (XX). This difference in chromosome number explains why hemophilia affects males and females differently.
A male who inherits one mutated X chromosome will have hemophilia, because he has no second X chromosome to carry a working copy of the gene. A female who inherits one mutated X chromosome is usually a carrier—she has a working copy on her other X chromosome, so her body can still make clotting protein, though sometimes at lower levels. A female would need to inherit the mutation on both X chromosomes to have hemophilia itself, which is rare.
Carriers often have no symptoms or only mild bleeding problems. However, some carriers experience symptoms closer to those of hemophilia because of a process called X-inactivation, in which one X chromosome in each cell is randomly silenced. If the silenced X happens to be the one carrying the working gene, that cell cannot make clotting protein. The more cells that silence the normal copy, the more symptoms a carrier may have.
What your parents' status means for your risk
If your mother is a carrier or has hemophilia, you have a 50 percent chance of inheriting the mutation from her. If your father has hemophilia, the situation is different: he will pass his X chromosome to all his daughters (making them carriers) and his Y chromosome to all his sons (who will not inherit hemophilia from him). Sons inherit their X chromosome from their mother, not their father.
If neither parent has hemophilia or is known to be a carrier, but you were diagnosed with hemophilia, a new mutation occurred in you. This spontaneous mutation is not inherited from your parents and does not mean your parents carry the gene. However, if you have hemophilia and plan to have children, the inheritance pattern applies to them: your daughters will inherit your X chromosome and become carriers, and your sons will inherit your Y chromosome and will not have hemophilia.
The inheritance pattern also matters for siblings. If your mother is a carrier, each of your siblings has the same 50 percent chance you did. If your father has hemophilia and you are male, your brothers did not inherit it from him (they got his Y chromosome instead), but they may have inherited it from your mother if she is a carrier.
Genetic testing and what results mean
A blood test can identify the specific mutation causing hemophilia in your family. Genetic testing looks for the faulty gene in your DNA and can confirm hemophilia diagnosis, identify carriers, and predict who in your family may be affected. Testing is most useful when someone in the family already has a hemophilia diagnosis, because then the lab knows exactly which mutation to look for in relatives.
If you test positive for the mutation, you have hemophilia or are a carrier depending on your sex and the test results. If you test negative, you do not carry that particular mutation. Genetic counselors—specialists trained in how genes are inherited—can help you understand what your results mean for you and your family and discuss the implications for future children.
Testing is not required to diagnose or treat hemophilia, and many people live with hemophilia without ever having genetic testing. However, testing can be valuable if you are planning to have children, if you want to know whether relatives are carriers, or if you want to understand your own risk more clearly.
Why some people with hemophilia have no family history
About one in three people diagnosed with hemophilia have no known family history of the condition. This happens when a new mutation occurs spontaneously in the egg or sperm cell that created them. The mutation was not present in either parent's body, so there was no way to predict or prevent it. Once the mutation occurred, it became part of that person's DNA in every cell.
A new mutation is just as real and just as permanent as an inherited one. The person has hemophilia and may pass the mutation to their children, even though their parents do not carry it. Genetic testing can sometimes determine whether a mutation is new or inherited by examining the parents' DNA, though this is not always possible.
Hemophilia in females: carriers and symptomatic carriers
Females with one mutated X chromosome are typically carriers and may have no bleeding symptoms at all. However, some carriers experience mild to moderate bleeding problems—nosebleeds, heavy periods, or prolonged bleeding after injury—because of uneven X-inactivation. These women are sometimes called symptomatic carriers.
A female with hemophilia (two mutated X chromosomes) is extremely rare and occurs only when both her mother and father carry or have the mutation. Her father would need to have hemophilia, and her mother would need to be at least a carrier. Females with hemophilia have the same clotting problems as males with hemophilia and receive the same treatment.
Carrier status matters for family planning. A carrier woman has a 50 percent chance of passing the mutation to each child. Her sons who inherit it will have hemophilia; her daughters who inherit it will be carriers. Genetic counseling before pregnancy can help carriers understand these odds and explore options.
How hemophilia mutations arise and change over time
Hemophilia mutations are changes in the DNA sequence of the gene responsible for making clotting factor VIII (in hemophilia A) or factor IX (in hemophilia B). These changes can be large deletions, small point mutations, or other alterations. Different families often carry different mutations, which is why genetic testing identifies the specific change in your family rather than testing for hemophilia in general.
Once a mutation is present in a person's body, it does not change or worsen over their lifetime. The mutation is the same in every cell that contains it. However, the severity of hemophilia symptoms can vary between people who carry the same mutation, partly because of differences in X-inactivation (in females) and partly because of other genetic and environmental factors.
Mutations do not spontaneously repair themselves, and hemophilia cannot be cured by changing your lifestyle or environment. Treatment focuses on replacing the missing clotting factor or helping your body use the factor you do have more effectively, not on fixing the underlying genetic mutation.
Frequently Asked Questions
Can I pass hemophilia to my children if I'm a carrier?
Yes. If you are a carrier female, each child has a 50 percent chance of inheriting the mutation. Sons who inherit it will have hemophilia; daughters will be carriers. If you are a male with hemophilia, all your daughters will be carriers and none of your sons will inherit hemophilia from you (they inherit your Y chromosome instead).
If my father has hemophilia, will I have it?
If you are male, no—you inherit the Y chromosome from your father, not his X chromosome. If you are female, you will be a carrier because you inherit his X chromosome. However, you may have mild symptoms because of X-inactivation. Your brothers could have hemophilia if your mother is a carrier or has hemophilia.
Does hemophilia skip generations?
Hemophilia does not skip generations in the sense that the mutation disappears and reappears. However, it may appear to skip a generation if a carrier female has no symptoms and is unaware she carries the mutation. Her sons who inherit it will have hemophilia, making it seem like the condition appeared suddenly.
Can two parents without hemophilia have a child with hemophilia?
Yes, if the mother is a carrier. A carrier mother has no symptoms but carries the mutation on one X chromosome. She has a 50 percent chance of passing it to each child. Additionally, a new mutation can occur spontaneously, resulting in hemophilia in a child whose parents do not carry the gene.
Should I have genetic testing if hemophilia runs in my family?
Genetic testing can confirm carrier status and help you understand your risk and your children's risk. It is most useful if you are planning to have children, if you want to know whether relatives are carriers, or if you want clarity about your own status. A genetic counselor can help you decide whether testing makes sense for your situation.