Haemophilia is most common in males because it's genes are on the X chromosome. But doesn't the Y chromosome determine if you are male or not?



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Hemophilia is also known as bleeder's disease. It is a sex-linked condition. It is linked to the X chromosome. Sex is determined by which sex chromosomes a person inherits. If someone has two X chromosomes or XX, they are female and if someone has an X and a Y chromosome, they are male.  However, it is the X chromosome that is important where hemophilia is concerned. Males only get one X chromosome, which is passed to them from their mothers. If the X has the normal gene, they will have normal blood clotting ability. If the X chromosome has the hemophilia gene, they will have the disease phenotype of hemophilia. Their Y chromosome plays no role in the inheritance of this condition. It passes from mother to son however because it is located on the X chromosome. Females have two X chromosomes. If they have one normal X and one X containing the hemophilia gene, they are called carriers. They don't actually have hemophilia. The hemophilia gene is recessive to the normal gene. However, for a female to actually get hemophilia, both of their X chromosomes would each have to have a hemophila gene. Thus, it is rare for females to get this recessive condition. 

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