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What Causes Hemophilia?

Hemophilia is an inherited disorder – typically passed on to boys through the X chromosome. But, hemophilia can also occur when there is no family history.

Remember how DNA ‘codes’ proteins, like clotting factors? When there is a change in the DNA sequence, called a mutation, the way proteins are made can be affected. Hemophilia is caused by a mutation in the gene responsible for producing clotting factors. The type of hemophilia a person has depends on which gene is disrupted. Researchers don’t fully understand why mutations like these happen.

How hemophilia is inherited

So, why are males much more likely to have hemophilia? It’s because hemophilia is passed on through the X chromosome. Males are ‘XY’ so they only have one X chromosome that they inherit from their mother. So, if his X chromosome contains the gene responsible for hemophilia, there is no ‘normal’ X chromosome to ‘override it.’ Females, on the other hand, have two X chromosomes (are ‘XX’), so there will usually be one ‘normal’ X chromosome to protect her from having severe hemophilia. It is very rare for a girl to be born with severe hemophilia.

What does being a carrier mean?

There are some women who have one hemophilia gene and, because of this, bleed longer than a normal person. Usually, women with one hemophilia gene are carriers. A carrier is a woman who may not show any symptoms, but carries the gene mutation for hemophilia and can pass it on to her children. Women who have a family history of hemophilia, or show mild symptoms of a bleeding disorder, may choose to be tested to see if they are a carrier.

The chances of passing on the hemophilia gene

There are many different ways the hemophilia gene can be passed on. Here are two scenarios:

Scenario 1 (see below): If a woman who is a carrier has a son with a man without the hemophilia gene, the son has a 50/50 chance of developing hemophilia. If they have a daughter, she will not have hemophilia, but has a 50% chance of being a carrier.

Scenario 2 (see below): If a man with hemophilia has children with a woman who is not a carrier, none of their sons will have hemophilia because the father passes along his normal Y chromosome and the mother passes on a normal X. However, all of the couple's daughters will be carriers.


Possible patterns of inheritance

Diagram showing how the hemophilia gene is passed on from parents to their children Scenario 1

Diagram showing how the hemophilia gene is passed on from parents to their children Scenario 2

Adapted from Centers of Disease Control: Hemophilia. Available at: http://www.cdc.gov/ncbddd/hemophilia/inheritance-pattern.html

A girl can be born with severe hemophilia only when:

  • A woman who is a carrier has a child with a man who has hemophilia – so both pass on a mutated X chromosome.
  • A woman who is a carrier has a child with a man who does not have hemophilia AND a second new spontaneous hemophilia mutation happens when the child is conceived.
  • A man who has hemophilia has a child with a woman who is not a carrier AND a second new spontaneous hemophilia mutation happens when the child is conceived.

These three situations are extremely rare.

Hemophilia when there is no family history

In about 3 cases in every 10, a son will be born to a family that has no history of hemophilia. There are two reasons why this might happen:

  • An unknown family history
    It is possible that hemophilia was in the family for generations, but went undetected because no son with hemophilia was born in living memory.
  • A ‘spontaneous’ genetic mutation
    Some cases of hemophilia are caused by a new, or spontaneous, genetic mutation. This means that a genetic problem unexpectedly happens in a person, for no known reason. This could happen either in the mother at the time she was conceived (making her a silent carrier), or in the child at the time of conception (so his mother is not a carrier). Researchers don’t fully understand why these mutations happen, and why they happen in some people and not others.

Because there isn’t always a history of it, hemophilia can affect any family. If you notice the signs and symptoms of a bleeding disorder after a child’s birth, talk to your doctor right away. Testing can be done to see if it is hemophilia.

XMYTH: Boys always inherit hemophilia from their mothers.

checkmarkFACT: In most cases, the gene responsible for hemophilia is passed from a mother her son at the time of conception. But, 3 out of 10 boys develop hemophilia without inheriting it from his mother and when there is NO family history.

Information Booklet on Mild Hemophilia

A booklet for people living with milder forms of hemophilia, developed by the Atlantic Hemophilia Nurses Group of the Canadian Association of Nurses in Hemophilia Care (CANHC).

Download Booklet on Mild Hemophilia

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