Everyone has the genes responsible for making factor VIII (8) and factor IX (9). These factors are necessary for blood to clot.
Haemophilia is caused by a change (often called a mutation or alteration) in the factor VIII or IX gene. In the community this gene change is sometimes called the ‘haemophilia gene’.
There are two types of haemophilia and they both have the same symptoms:
- Haemophilia A is the most common and is caused by not having enough of clotting factor VIII (8)
- Haemophilia B is caused by not having enough of clotting factor IX (9)
Women and girls with an alteration on their factor VIII or IX gene are often described as ‘carrying the gene‘ or as a ‘haemophilia carrier‘.
FACTOR LEVELS AND BLEEDING SYMPTOMS
Most often a female who is a ‘haemophilia carrier‘ will have normal clotting factor levels and will not have symptoms of a bleeding disorder.
However, around 20-30% of females with the gene change have reduced factor levels and bleeding symptoms. They may have factor levels low enough to be classified as having the medical condition haemophilia, usually mild haemophilia (5-40% of normal clotting factor). In a few rare cases girls and women can have extremely low factor levels and have severe haemophilia.
Some females with factor levels in the ‘normal’ range (between 40% to 50%) may also experience abnormal bleeding related to haemophilia. These females are diagnosed as ‘symptomatic haemophilia carriers‘.
INHERITANCE
If you are a female who carries the gene for haemophilia or you are a male with haemophilia, you will have an alteration in your factor VIII (8) or IX (9) gene. This altered gene may be passed on to your children.
If you are a female who carries the gene change, there is a 50% chance with each of your pregnancies that you will pass the gene change onto your baby:
- If you have a son who inherits the gene, he will have haemophilia.
- If you have a daughter who inherits the gene, she will carry the gene change too.
If you are a male with haemophilia:
- All (100%) of your daughters will inherit the gene change from you and will be haemophilia carriers.
- None of your sons will have haemophilia.
MORE INFORMATION
Contact details of Haemophilia Treatment Centres in Australia are available on the HFA website.
If you’d like to know more about haemophilia A and B download our Guide to haemophilia testing in women and girls here. This contains more information on Haemophilia A and B including:
- What does it mean to have the gene change for haemophilia?
- Inheritance
- Bleeding symptoms
- Female bleeding patterns
- Genetic and factor level testing
- Your diagnosis and treatment
For more information on bleeding disorders in young women read our full Female Factors resource here