What is hemophilia?
Hemophilia is a rare hereditary (inherited) bleeding disorder in
which blood cannot clot normally at the site of a wound or injury.
The disorder occurs because certain blood clotting factors are
missing or do not work properly. This can cause extended bleeding
from a cut or wound. Spontaneous internal bleeding can occur as
well, especially in the joints and muscles. Hemophilia affects
males much more often than females.
There are two types of inherited hemophilia:
- Type A, the most common type, is caused by a
deficiency of factor VIII, one of the proteins that helps blood to
form clots.
- Type B hemophilia is caused by a deficiency of
factor IX.
Although hemophilia is usually diagnosed at birth, the disorder
can also be acquired later in life if the body begins to produce
antibodies that attack and destroy clotting factors. However, the
noninherited or acquired type of hemophilia is very rare.
What causes hemophilia?
The genes that regulate production of factors VIII and IX are
found on the X chromosome only. Hemophilia is caused by an abnormal
gene on the X chromosome. If a woman carries the abnormal gene on
one of her chromosomes (females have a pair of X chromosomes), she
will not have hemophilia herself, but she will be a carrier of the
disorder. That means that she can pass the gene for hemophilia on
to her children. There is a 50% chance that any of her sons will
inherit the gene and will be born with hemophilia. There is also a
50% chance that any of her daughters will be carriers of the gene,
without having hemophilia themselves.
It is very rare for a girl to be born with hemophilia, but it
can happen if the father has hemophilia and the mother
carries the gene for hemophilia. Their daughter will then have the
abnormal gene on both of her X chromosomes.
In about 20% of all cases of hemophilia, the disorder is caused
by a spontaneous gene mutation.
How common is hemophilia?
Hemophilia is an extremely rare disorder: about 18,000 people in
the U.S. have been diagnosed. Some 400 babies in the United States
are born with hemophilia every year. About 9 out of 10 hemophiliacs
have type A hemophilia. Around 60 to 70% of people with hemophilia
have the severe form of the disorder, about 15% have the moderate
form, and the rest have mild hemophilia. Hemophilia can occur in
all races and ethnic groups.