How is haemophilia passed on?
Haemophilia is an inherited condition and runs in families.
Haemophilia is caused by a mutation or alteration in the gene making factor VIII (8) or factor IX (9).
- these genes are called the F8 gene and F9 gene
- all females with the gene alteration are referred to as ‘carriers’, because they ‘carry’ the gene alteration.
This gene alteration is passed down from parent to child through generations. Men with haemophilia will pass the gene alteration on to their daughters but not their sons. Women who are carriers can pass the gene alteration on to their sons and daughters. Sons with the gene alteration will have haemophilia. Daughters with the gene alteration can have normal or reduced factor VIII or factor IX levels. Some will not have bleeding symptoms, while others can have symptoms and can have haemophilia.
No family history of haemophilia
In about one third of people born with haemophilia, there is no history of the disorder in the family. This happens when a new alteration in the genetic code of the F8 or F9 gene occurs by chance in an egg cell or sperm cell. The child who is conceived will have haemophilia or be a carrier and can pass the gene alteration on to their children. It is often called a new or spontaneous mutation.
Sometimes this gene alteration has occurred a generation or two earlier and the family has not known about it until they are tested.
Once haemophilia appears in a family the gene alteration is then passed on from parents to children following the usual pattern for haemophilia. Family members should seek genetic counselling and testing if there is someone in the family who has haemophilia.
Haemophilia inheritance pattern
This diagram shows the typical experience of genetic inheritance in haemophilia. It is explained in more detail below, along with some notes about gender diversity.
Cells and chromosomes
Every person has millions of cells that make up their body. At the centre of each cell are 46 chromosomes arranged in pairs. The chromosomes are packages in cells that contain the person’s genetic information or ‘genes’ and determine the person’s individual characteristics, such as the colour of their eyes or hair.
Everyone is born with ‘sex’ chromosomes. The X and Y sex chromosomes help to determine haemophilia inheritance patterns.
How do children inherit X and Y sex chromosomes from their parents?
- Individuals who are assigned ‘female’ at birth typically have two X chromosomes and receive one from each parent.
- Individuals who are assigned ‘male’ at birth typically have one X chromosome, which they receive from one parent, and one Y chromosome, which they receive from the other.
Gender affirming medical care after birth, eg hormones or surgery that some trans or gender diverse people may use to affirm their gender, will not change a person’s sex chromosomes or influence haemophilia inheritance patterns.
The Haemophilia Treatment Centre can support gender diverse people with haemophilia with:
- help to understand their individual bleeding and inheritance patterns
- a personalised treatment plan
- clear advice on management of their bleeding episodes, for example, on the ABDR patient card, to make sure they receive appropriate care.
The following sections explain the typical experience of genetic inheritance in haemophilia, but it can be more complex in some individuals. In these cases, the Haemophilia Treatment Centre will provide individualised advice and can refer the person to genetic specialists, if appropriate.
The genes for making factor VIII (8) and IX (9) are located on the X chromosome.
Any male or female with an alteration in their F8 or F9 gene can pass it on to their children.
X chromosomes and blood clotting
Because females have two copies of these genes, one inherited from each parent, women with an alteration in their F8 or F9 gene usually have another copy that is not altered. This helps factor VIII and IX to work properly in the blood clotting process and is often enough to control bleeding.
However, the Y chromosome does not have a gene to produce these factors. Males with a F8 or F9 gene alteration on their X chromosome do not have another factor VIII or IX gene to help with blood clotting and will have haemophilia.
Male inheritance pattern
- If a male with haemophilia has children, all his daughters will be haemophilia carriers because he will pass the F8 or F9 gene alteration on to them on his X chromosome.
- None of his sons will have haemophilia as his unaffected Y chromosome is passed on to them and they receive their X chromosome from their mother.
Female inheritance pattern
Females can carry the gene alteration causing haemophilia even if they don’t have symptoms.
A woman who carries the gene alteration can pass it on to both her sons and her daughters.
It is helpful to remember that genetics does not have a memory.
With each pregnancy:
- there is a 50% chance her male baby will have haemophilia
- and a 50% chance her female baby will be a carrier.