Hemophilia A is caused by mutations in the F8 gene, which provides instructions for making a protein called Factor VIII, essential for blood clotting. The condition affects roughly 1 in 5,000 males and ranges from mild to severe depending on how much the mutation disrupts Factor VIII production. In a smaller number of cases, hemophilia A develops later in life when the immune system mistakenly attacks Factor VIII, even without any genetic mutation.
The Role of Factor VIII in Clotting
When you’re injured, your body launches a chain reaction called the coagulation cascade, involving about 20 different proteins that work together to form a blood clot. Factor VIII is one of those proteins. It acts as a helper in the middle of the cascade, accelerating a critical step where other clotting factors combine to seal off damaged blood vessels.
Without enough Factor VIII, this chain reaction stalls. Blood can’t clot properly, and bleeding continues longer than it should. In severe cases, bleeding can start spontaneously, even without an obvious injury, particularly in joints and muscles.
Mutations in the F8 Gene
More than 1,300 different mutations in the F8 gene have been identified so far. Some are small changes to a single DNA building block. Others involve larger deletions or insertions of genetic material. The most common mutation in people with severe hemophilia A is an inversion, where a large segment of the F8 gene gets flipped around and rearranged. This essentially shuts down Factor VIII production almost entirely.
The type of mutation determines how much Factor VIII your body can still produce, which directly controls how severe the condition is:
- Severe hemophilia A: Less than 1% of normal Factor VIII activity. Spontaneous bleeding episodes are common, often into joints and soft tissues.
- Moderate hemophilia A: Between 1% and 5% of normal activity. Bleeding is excessive after minor injuries or trauma but rarely spontaneous.
- Mild hemophilia A: Between 6% and 40% of normal activity. Prolonged bleeding typically only occurs after surgery, dental work, or significant injury. Some people with mild hemophilia don’t discover the condition until adulthood.
Large inversions and deletions tend to produce severe disease because they destroy the gene’s ability to make functional protein. Smaller point mutations often leave the protein partially functional, resulting in moderate or mild forms.
How Hemophilia A Is Inherited
The F8 gene sits on the X chromosome, which means hemophilia A follows an X-linked inheritance pattern. Males have one X and one Y chromosome, so a single mutated copy of the F8 gene is enough to cause the condition. Females have two X chromosomes, so a working copy on the second X chromosome typically compensates for a mutated one.
This is why hemophilia A overwhelmingly affects males, while females are usually carriers. The inheritance math breaks down clearly depending on which parent carries the mutation.
When the Mother Is a Carrier
If a mother carries one mutated F8 gene and the father is unaffected, each son has a 50% chance of inheriting hemophilia A. Each daughter has a 50% chance of becoming a carrier herself. Overall, there’s a 25% chance with each pregnancy that the baby will be a son with hemophilia and a 25% chance it will be a carrier daughter.
When the Father Has Hemophilia A
A father with hemophilia passes his X chromosome to every daughter, making all of his daughters carriers. He passes his Y chromosome to every son, so none of his sons inherit the condition from him. If the mother doesn’t carry a hemophilia allele, no children will actually have the disease, but every daughter will carry the gene and could pass it to her own sons.
Cases With No Family History
About 30% of people diagnosed with hemophilia A have no family history of the condition. These cases arise from spontaneous, or “de novo,” mutations that occur during the formation of the egg or sperm, or very early in embryonic development. The mutation is new to that individual and wasn’t present in either parent’s blood cells.
This is an important point because many families assume hemophilia must run in the family for a child to be affected. A significant portion of cases appear without warning in families that have never encountered the condition. Once the new mutation exists, though, it can be passed to future generations following the standard X-linked pattern.
Acquired Hemophilia A
Not all hemophilia A is genetic. In rare cases, the immune system begins producing antibodies that attack and neutralize Factor VIII. This is called acquired hemophilia A, and it can develop in people who previously had completely normal clotting.
About half of people who develop acquired hemophilia A have an underlying immune system disorder, such as an autoimmune disease or a type of blood cell cancer. The condition is also linked to pregnancy, where some women develop antibodies against Factor VIII in the weeks after giving birth. In fact, the majority of acquired hemophilia cases in people between ages 20 and 30 are in women, largely because of pregnancy-related immune changes. Drug reactions and aging are other recognized triggers, though in many cases no clear cause is found.
The antibodies in acquired hemophilia latch onto specific parts of the Factor VIII protein, blocking it from interacting with other clotting factors or from binding to the surfaces where clotting takes place. The result is the same as the genetic form: prolonged, sometimes dangerous bleeding. But because it strikes suddenly in adults who have no prior bleeding history, it is often diagnosed late.
How the Cause Determines Severity
Whether hemophilia A is genetic or acquired, the core problem is identical: not enough working Factor VIII in the blood. But the cause shapes the experience significantly. Genetic hemophilia A is present from birth, diagnosed in childhood (often when a child starts crawling or walking and bruises easily), and managed with lifelong treatment. Severity stays consistent over a person’s lifetime because the underlying mutation doesn’t change.
Acquired hemophilia A, by contrast, appears suddenly, often in older adults, and its severity can fluctuate depending on how many antibodies the immune system is producing. It can sometimes resolve if the underlying trigger is treated or if the immune response is suppressed.
For genetic cases, the specific mutation matters beyond just severity classification. Certain mutations, particularly large inversions and deletions, carry a higher risk of the immune system reacting against replacement Factor VIII during treatment, developing what are called “inhibitors.” This complicates management and is one reason genetic testing is done early after diagnosis, not just to confirm the condition but to anticipate how treatment may go.