Hemophilia is caused by a missing or faulty gene that tells your body how to make blood clotting proteins

Hemophilia develops because of a problem in your DNA — the instruction manual your cells use to build proteins. Specifically, the gene responsible for making clotting factor VIII (in hemophilia A) or clotting factor IX (in hemophilia B) is either missing parts, damaged, or not working correctly. Without these proteins, your blood cannot form clots the way it should, so even small cuts or injuries bleed longer than normal.

You are born with hemophilia because you inherited this faulty gene from one or both of your parents. The condition is not something you catch or develop later in life — it is present from birth, even if you do not notice symptoms until you have an injury or a medical procedure.

Key Takeaways

  • Hemophilia is caused by a gene mutation that prevents your body from making enough clotting factor VIII or IX, the proteins needed to stop bleeding.
  • The condition is inherited from parents and present from birth, passed down through families in a pattern linked to the X chromosome.
  • Hemophilia A is more common than hemophilia B, and males are affected far more often than females because of how the gene is inherited.
  • A single gene mutation can vary in severity — some people have very little clotting protein and bleed easily, while others have partial amounts and bleed only with injury or surgery.

How the faulty gene gets passed down in families

The gene for clotting factors sits on the X chromosome, one of the two sex chromosomes. Males have one X chromosome (and one Y), while females have two X chromosomes. This difference in chromosome number is why hemophilia inheritance works differently for men and women.

If your mother carries one faulty copy of the gene on one of her X chromosomes, she typically has one working copy on her other X chromosome, so she usually does not have severe bleeding problems herself. However, she has a 50 percent chance of passing the faulty gene to each child. Sons who inherit it will have hemophilia. Daughters who inherit it will be carriers like their mother.

If your father has hemophilia, he will pass his faulty X chromosome to all of his daughters (making them carriers) but to none of his sons, because sons inherit the Y chromosome from their father instead. This is why hemophilia is far more common in males than females.

What happens when the clotting protein is missing or broken

Blood clotting is a chain reaction. When you cut yourself, your body needs to build a plug to seal the wound. Clotting factors are proteins that trigger each step of this process. Factor VIII and factor IX are two of the most important links in that chain.

When the gene for one of these factors is faulty, your body either makes very little of the protein or makes a protein that does not work properly. Without enough working clotting factor, the chain reaction stalls. Your blood can still start to clot, but it cannot finish the job quickly or completely. This is why people with hemophilia bleed longer after cuts, have bruising from minor bumps, or experience bleeding into joints and muscles even without an obvious injury.

The severity depends on how much clotting factor your body can still make. Some people with hemophilia have almost none and bleed spontaneously (without any injury). Others have partial amounts and bleed mainly after trauma or surgery. This variation comes from the specific mutation in the gene — different mutations cause different degrees of protein loss.

Why some people have hemophilia and others in the same family do not

Because the faulty gene is inherited, hemophilia runs in families. However, not everyone in a family will have the condition or even carry the gene. The inheritance depends on which parent carries the mutation and which chromosomes each child inherits.

A mother who is a carrier has a 50-50 chance with each pregnancy. A father with hemophilia will pass the gene to all daughters but no sons. If neither parent has the gene or is a carrier, a child cannot inherit hemophilia — unless a new mutation occurs spontaneously, which happens in roughly one-third of hemophilia cases. In these situations, the faulty gene appears for the first time in that person, with no family history.

Spontaneous mutations: hemophilia with no family history

About one in three people with hemophilia have no family history of the condition. This happens when a new mutation occurs in the egg or sperm cell that created them, or very early in their development. The faulty gene was not inherited from a parent — it arose by chance.

When a spontaneous mutation occurs in a male, he will have hemophilia. If it occurs in a female, she will be a carrier. From that point forward, the faulty gene can be passed to the next generation just like any inherited mutation. A woman with a spontaneous mutation has a 50 percent chance of passing it to each child.

How doctors confirm the genetic cause

Doctors diagnose hemophilia by measuring clotting factor levels in a blood test. If the levels are low, they can order genetic testing to identify the specific mutation in the factor VIII or factor IX gene. This test confirms the diagnosis and can help predict severity and guide treatment decisions.

Genetic testing can also identify carriers — people who have one faulty copy of the gene but may not have symptoms. Carriers sometimes have mild bleeding symptoms, and knowing carrier status is important for family planning and understanding the risk to future children.

Frequently Asked Questions

Can hemophilia develop later in life if I was not born with it?

No. Hemophilia caused by a genetic mutation is present from birth. However, acquired hemophilia — a rare condition where the body develops antibodies against clotting factors — can develop in adults. This is a different condition with different causes and is not inherited.

If my mother is a carrier, will I definitely have hemophilia?

Not definitely. If your mother is a carrier, you have a 50 percent chance of inheriting the faulty gene with each pregnancy. Sons who inherit it will have hemophilia; daughters who inherit it will be carriers. Sons who do not inherit it will not have the condition.

Can a genetic mutation be fixed or cured?

Current treatments do not fix the underlying gene mutation. Instead, they replace the missing clotting factor through infusions. Gene therapy is being studied in clinical trials and may offer a different approach in the future, but it is not yet standard treatment.

Why do males get hemophilia more often than females?

Males have only one X chromosome, so one faulty copy of the gene causes hemophilia. Females have two X chromosomes, so they usually have one working copy that can partially compensate. A female would need two faulty copies — one on each X chromosome — to have severe hemophilia, which is very rare.

If I am a carrier, can I pass hemophilia to my children?

Yes. If you are a carrier, you have a 50 percent chance of passing the faulty gene to each child. Sons who inherit it will have hemophilia; daughters will be carriers. Genetic counseling can help you understand the risks before pregnancy.