Haemophilia is caused by an inherited genetic mutation, which mainly affects males.
A
genetic mutation is a permanent alteration in the DNA sequence that makes up a
gene. This means that some of the body's processes won't work in the normal
way.
The
type of mutation determines whether a family will experience mild, moderate or
severe symptoms.
Chromosomes
are strands of DNA. They contain a detailed set of instructions that control a
wide range of factors, including how the body's cells develop and the baby's
sex.
There
are two types of sex chromosomes: the X chromosome and the Y chromosome. All
humans have a pair of sex chromosomes. Men have an XY pair and women have an XX
pair.
Boys
inherit their X chromosome from their mother and their Y chromosome from their
father. Girls inherit an X chromosome from each parent.
Haemophilia
is inherited through a mutation in the X chromosome.
The
chances of a child inheriting the haemophilia mutation depend on which of their
parents has the mutated gene.
Only the mother is affected
If
a woman with the mutated X chromosome and an unaffected man have a baby,
there's a:
one
in four chance of having an unaffected baby boy
one
in four chance of having a baby boy with haemophilia
one
in four chance of having an unaffected baby girl
one
in four chance of having a baby girl with one affected chromosome
In
the last situation, the girl becomes a carrier of the mutated gene. This means
she can pass it on to her children, but won't usually have any severe symptoms
of haemophilia herself. However, some female carriers sometimes have bleeding
problems, such as heavy periods.
Only the father is affected
If
a man with haemophilia has a son with an unaffected woman, there's no chance
the boy will get haemophilia. This is because he always inherits his X
chromosome from his mother.
However,
any daughters the man has will become carriers of the mutated haemophilia gene
and may pass it on to their children.
Both parents are affected
If
a woman with the mutated chromosome and a man with haemophilia have a baby,
there's a:
one
in four chance of having an unaffected baby boy
one
in four chance of having a baby boy with haemophilia
one
in four chance of having a baby girl who is a carrier of haemophilia
one
in four chance of having a baby girl with haemophilia
This
means it's possible for a female to have haemophilia, although it's very rare.
No family history
In
some cases, a boy is born with haemophilia even though there's no family
history of the condition.
In
such cases, it's thought the mutation developed spontaneously in the boy's mother,
grandmother or great-grandmother, but until then a male member of the family
had never inherited the mutated gene.
Although
family history is a strong indicator, some studies have shown there's no known
family history of haemophilia in up to one-third of new cases.
