The cause of hemophilia is a mutation to one or more genes that encodes clotting factors. These genes are translated into proteins, and these proteins are essential for coagulation. Having a defective gene or having it depleted leads to a faulty or absent protein and subsequent coagulation problems. These clotting genes can be found in the X chromosome found in males (XY) and females (XX). Thus, this is a sex-linked disease.
Hemophilia is a recessive disorder, which means that you need to have both X chromosomes taken to suffer the disease. Unluckily for males, they only have one of them, and they will have hemophilia whenever their X chromosome copy has the mutation. In women, having two X chromosomes allow them to be carriers of the disease without any symptom. The other copy of chromosome X does the job when it is intact, and they will only have hemophilia when both X chromosomes are affected by the disease. That’s why men are more likely to have hemophilia, and women would only suffer from the disease if they are born from men with hemophilia and carrier women who may or may not have the same condition.
Females can be carriers of hemophilia without having any symptoms. They are often unaware of their defective gene until they pass the mutated chromosome on to their offspring.
Hemophilia genes run in families, but sometimes they are not entirely evident. There are families with no prior history of hemophilia cases because they mainly have female carriers, and males in the family are not affected. But then, the first relative with hemophilia shows up, revealing the inheritance of the mutation in the X chromosome.
In some cases, hemophilia can be further complicated by the development of inhibitory antibodies to clotting factors. When this is the case, lacking the clotting factor is not the only problem, and treatment becomes more difficult and expensive.
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