Different types of haemophilia
Haemophilia A is often called classical haemophilia, and is the most common type of haemophilia affecting one in 5,000 males. Haemophilia A is due to a deficiency of factor VIII (factor 8).
Haemophilia B is often called Christmas Disease, named after the first person diagnosed with the condition. Haemophilia B is caused by a deficiency in factor IX (factor 9) and affects one in 30,000 males.
Both disorders cause bleeding into the joints, muscles and other soft tissues, as a result of trauma or injury. In addition bleeding often happens spontaneously in the severe type of the disorder. Because both types of haemophilia share the same symptoms and inheritance pattern, a blood test is the only way to identify whether someone has haemophilia A or B. It is important to know which factor is lacking, so that the correct treatment can be given.
Levels of haemophilia
Haemophilia A and B are classified as mild, moderate or severe. This depends on the level of clotting factor in the blood.
Mild haemophilia - Blood clotting levels between 5% and 25%.
Moderate haemophilia - Blood clotting levels between 2% and 5%.
Severe haemophilia - Blood clotting levels less than 1%.
Carriers - Carriers of the haemophilia gene can have average levels of clotting factor VIII or IX at 25% to 50% or even lower. Many carriers of the haemophilia gene will have normal factor levels (50% or above).
For example, someone with less than 1% of clotting factor in their blood is classified as having severe haemophilia.
Someone with mild haemophilia will usually have few problems and will generally only need treatment for their condition after tooth extraction, surgery or after an injury.
Someone with moderate or severe haemophilia can have bleeds after a minor bump or knock.
People with severe haemophilia have spontaneous bleeds as well as bleeds from trauma and injury. Spontaneous bleeding in moderate and mild haemophilia is rare. Frequent bleeding into joints, muscles and soft tissues is very painful and disabling.
Bleeding, particularly after a bad head injury, may be life threatening if not treated effectively.
One in three children with haemophilia is born into a family with no history of the condition.
Understanding how inheritance patterns work Haemophilia is a genetic condition
There is usually a history of haemophilia in the family, but one in three children with
haemophilia is born into a family with no history of the condition.
Very rarely, it is possible for someone to develop haemophilia. This is known as acquired haemophilia.
Haemophilia is a ‘sex linked recessive’ condition. This means that while only males have the condition, it is passed through the female line in the family.
It is rare for a female to inherit true haemophilia. For this to happen her father would have to have haemophilia and her mother would have to carry the haemophilia gene. Many females who are carriers of haemophilia can have very low clotting factor levels and show signs of
‘mild’ haemophilia.
The inheritance pattern can be difficult to grasp at first. But it is very important to understand it properly so that families with a history of haemophilia can get tailor-made advice and support.
How is it inherited?
The cells which make up our bodies are programmed with instructions from our mothers and fathers. These instructions are called genes and they hold the genetic instructions for life. The genes are carried on structures called chromosomes. Everyone has 46 chromosomes in their
body. Two of the chromosomes determine the sex of a baby. They are the X and Y
chromosomes.
Females have two X chromosomes (XX); males have an X and a Y (XY).
A child will inherit a chromosome from each parent. Males get an X chromosome from their mother and a Y from their father. Females get an X chromosome from each parent. The genes for clotting factors VIII and IX are on the X chromosome. There is no gene for clotting on the Y chromosome. Only one of the female’s X chromosomes can be affected by the faulty haemophilia gene.
In most inheritance family patterns, therefore, the other non-affected X chromosome compensates in the production of clotting factor VIII or IX.
Inheritance pattern if the mother carries the haemophilia gene and the father does not have haemophilia
• A male child will have a 50:50 chance of having haemophilia, as he will inherit one of his mother’s X chromosomes. If it is the faulty one he will inherit haemophilia.
• A female child will have a 50:50 chance that she will carry the haemophilia gene, as she will inherit an X chromosome from her mother.
Inheritance pattern if the mother is not a haemophilia carrier, but the father has haemophilia
• A female child will be a carrier, as she will have a normal X chromosome from her mother and the affected X chromosome from her father. Daughters of a man with haemophilia are known as ‘obligate carriers’; in other words, there is no way that they cannot be
carriers of haemophilia.
• A male child will not be affected, as he will inherit his father’s normal Y chromosome and his mother’s normal X chromosome. The son cannot pass haemophilia on to future generations.
Inheritance pattern if the mother carries the haemophilia gene and the father has haemophilia
• A female child can inherit haemophilia, but this is quite rare. Females do not usually have haemophilia because one of their X chromosomes works effectively. Some female carriers do
have reduced factor levels and may need factor replacement for surgery, tooth extraction or following childbirth.
The severity of haemophilia remains constant in a family. In other words, if there
is a family history of mild haemophilia, a carrier of haemophilia from that family can
only pass on a mild form of haemophilia.
It is possible for the haemophilia gene to remain hidden through several generations of a family if it has only been passed on to females.
The risk of having a child with haemophilia is the same for each pregnancy.
INHERITANCE DIAGRAM
Case A - Healthy Man & Healthy Woman
Man Woman
X Y X X
Male Child / Female Child will not have the gene.
Case B - Man with haemophilia & Healthy Woman
Man Woman X' Y X X
Female child will be Haemophilia carrier as she will inherit X' from Father (he can contribute only one X which is X') and normal X from Mother.
Male child will not have Haemophilia as he will inherit Y from the Father and normal X from Mother
Case C - Healthy Man & Carrier Woman
Man Woman X Y X' X Female child has a 50% chance of being a carrier, depending on whether X or X' is donated by Mother Male child has a 50% chance of being Haemophiliac, depending on whether X or X' is donated by Mother
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