The short answer
If a mother carries a gene change for hemophilia A or B and the father does not, each son has a 50% chance of having hemophilia and each daughter has a 50% chance of inheriting the change. Counting all pregnancies together, that is roughly a 25% chance of a son with hemophilia, a 25% chance of a daughter who carries it, and a 50% chance of a child who does not inherit it at all. If instead it is the father who has hemophilia, none of his sons inherit it and all of his daughters do.
Start with which parent carries it
| The family situation | Sons | Daughters |
|---|---|---|
| Mother carries the change, father does not | Each son has a 50% chance of having hemophilia | Each daughter has a 50% chance of inheriting the change |
| Father has hemophilia, mother does not carry it | No son inherits it from him | Every daughter inherits his changed X |
If both parents carry a change, the possibilities are different again — a daughter can then inherit a changed copy from both sides. That is much less common and needs its own calculation.
Why hemophilia follows the X chromosome
Hemophilia A is usually caused by a change in a gene called F8, and hemophilia B by a change in F9. Both sit on the X chromosome.
A mother with one changed X and one usual X passes one of them to each child. A son who inherits the changed one has no second X to make up for it, so he has hemophilia. A daughter who inherits it also has an X from her father — but she may still have low clotting-factor levels and bleeding symptoms.
| Child | Chance of inheriting the mother’s changed X | What usually follows |
|---|---|---|
| Son | 50% | Has hemophilia |
| Daughter | 50% | Carries the change; bleeding symptoms range from none to significant |
Why you also see the figure 25%
Both figures are correct — they answer different questions.
Half of pregnancies are boys, and half of those inherit the changed X:
1/2 (a son) × 1/2 (he inherits it) = 1/4
So each pregnancy has about a 25% chance of producing a son with hemophilia, and about a 25% chance of producing a daughter who carries it. The other half of children inherit neither.
Fifty per cent is the chance for a son. Twenty-five per cent is the chance across all pregnancies.
If it is the father who has hemophilia
He gives his Y to every son and his X to every daughter. So, assuming the mother does not carry a change:
- none of his sons inherit hemophilia from him;
- all of his daughters inherit his changed X.
This all-daughters-and-no-sons pattern is one of the clearest signs of X-linked inheritance there is.
“Carrier” does not mean “unaffected”
The word carrier can make it sound as though nothing happens medically. For hemophilia that is not reliable.
Some women and girls who carry the change have heavy periods, bruise easily, bleed for a long time after dental work or surgery, or bleed heavily after childbirth. Some have clotting-factor levels low enough to be diagnosed with hemophilia themselves.
Anyone known or suspected to carry the family change should have their own bleeding history taken and their factor levels measured. It matters for everyday care — not just for pregnancy.
How to find out whether you carry it
The best starting point is the genetic report of the relative who has hemophilia. If it names the exact F8 or F9 change, everyone else in the family can be tested for that one thing.
A normal clotting-factor level does not rule it out, because levels vary a lot. Family history can mislead too: when the first affected boy in a family has a brand-new gene change, there is nothing earlier in the family to find.
Once the family’s change is known, a genetics or haemophilia service can explain the testing available before or during a pregnancy. The first practical step is always the same: replace “hemophilia runs in the family” with the exact type, the gene change and a test result.
Common questions
- Can a father with hemophilia pass it to his son?
- No. He gives his son a Y chromosome, and the hemophilia gene is on his X. That X goes to every one of his daughters instead.
- Do all daughters of a man with hemophilia inherit it?
- Yes. As long as the mother does not also carry a change, every daughter inherits his changed X and no son does.
- Can women and girls have bleeding symptoms?
- Yes. Women and girls who carry a hemophilia change can have low clotting-factor levels, heavy periods, easy bruising or prolonged bleeding after surgery or childbirth. Some meet the criteria for hemophilia themselves.
- Can a blood test rule out being a carrier?
- Not on its own. Clotting-factor levels overlap between carriers and non-carriers, so a normal level does not settle it. Testing for the exact family change in the F8 or F9 gene does.