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Hemophilia
What causes it?
Hemophilia is a genetic (inherited) disorder, which means that it can be passed from one generation to the next. It is also a sex-linked disorder, which is why it appears mostly in males. A brief review of genetics will help make clear why few females have hemophilia.
Half of each human?s 46 chromosomes are contributed by each parent. Two of these chromosomes, known as X and Y, determine an individual?s gender. Females have two X and no Y chromosomes, while males have one of each. Each chromosome contains hundreds of genes, which determine an individual?s characteristics such as eye color or height. The genes responsible for the production of clotting factors VIII and XI are located on the X chromosome. Since males have only one X chromosome, a defective clotting gene on it means that the individual will have hemophilia. Normal genes for clotting factors dominate defective clotting genes, however. Therefore, because females have two X chromosomes, they have two sets of genes for producing clotting factors VIII and IX. If one set is abnormal, the other set can make up for the deficit. Only a female who has two sets of defective clotting genes will have hemophilia. A woman who has one X chromosome with abnormal activity is said to be a carrier. While all of her children may receive the functioning clotting factor genes from her, she may pass the defective X chromosome to a son, who will have hemophilia. If the daughter of a carrier receives the defective gene, she will also carry it and possibly pass it to her children.
About one-third of patients with hemophilia have no family history of the condition. For these individuals, the cause appears to be a genetic mutation, which means that some event during their pre-birth development altered one or more clotting factor genes.
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What causes it?
