Why are female bleeding patterns different?
Women and girls may wonder why their bleeding pattern is different to the males in their family.
- Males with haemophilia in the same family will nearly always have the same severity – for example, a grandfather and a grandson will both have severe haemophilia.
- Factor levels in females who are carriers are unpredictable and can vary even within the same family.
Nur and Fatimah's story
Nur and Fatimah are sisters who are both haemophilia carriers.
Nur has low factor levels and mild haemophilia.
Fatimah has normal factor levels and no symptoms.
Their father has severe haemophilia.
Nur and Fatimah have inherited the same ‘family’ gene alteration that their father has, but this gene alteration causes severe haemophilia in males.
Both Nur and Fatimah can pass on the gene alteration to their children.
- Their male children who inherit the gene alteration will have severe haemophilia.
- Their female children who inherit the gene alteration may have normal or low factor levels.
Why are factor levels unpredictable in females?
To understand factor levels in females who have a gene alteration for haemophilia, we need to look more closely at X chromosomes. Interested? Read on!
Haemophilia is due to a problem in one of the genes on the X chromosome – either the factor VIII (F8) gene or the factor IX (F9) gene.
A male with haemophilia (like Nur and Fatimah’s father) has an X chromosome with an F8 or F9 gene problem.
Because he is male, he only has one X chromosome and so he has haemophilia.
Affected females (like Nur and Fatimah) have inherited the X chromosome with the factor gene problem from one parent (eg, their father).
However, females inherit two X chromosomes, one from each parent.
Usually their other parent will have a normal X (like Nur and Fatimah’s mother).
So, although they have a problem with a factor gene on one X, affected females (like Nur and Fatimah) still have a second normal factor gene on their other X.
What happens to X chromosomes in females?
This is where it gets a bit more complicated.
In a female’s body, only one of her two X chromosomes is needed in each of her cells.
The other X chromosome is permanently ‘switched off’ or inactivated.
This process is called X-inactivation.
X-inactivation is a random natural process that happens in all females before birth.
How does X-inactivation affect factor levels?
If a female is affected by haemophilia, she usually has one normal X chromosome and one problem X in each cell.
In some of her cells the normal X will be switched off.
In the rest of her cells the problem X will be switched off.
This switching off process is random, so it is not possible to predict how many of her cells will have the normal X inactivated.
How much factor each cell makes depends on which X chromosome is switched off.
If more than half of a haemophilia carrier’s normal X chromosomes are switched off, most of her cells might not be able to produce clotting factor properly. She could have low clotting factor levels and bleeding problems.
This is why two sisters who both carry the gene alteration for haemophilia can have different clotting factor levels.
50% of Fatimah’s cells have an F8 or F9 gene that works properly.
This is enough for her blood to clot normally.
Only 30% of Nur’s cells have an F8 or F9 gene that works properly.
This is why her blood doesn’t clot well and she has mild haemophilia.
Why do some females have very low factor levels?
When females have very low factor levels, like the levels in moderate or severe haemophilia, this may sometimes be caused by other rare genetic conditions, for example, having two bleeding disorders or a different genetic disorder. Or in very rare cases, it could be that she has had the majority of her normal X chromosomes inactivated or inherited factor gene problems from both her parents.
This can be very complex and the girl or woman would need to have specialised advice, genetic counselling and genetic testing in liaison with a Haemophilia Treatment Centre and/or a Clinical Geneticist.