born with and has for life. The gene that causes hemophilia is carried on the X chromosome. It is called a sex-linked disorder because that is the chromosome that determines a person’s sex.
How Is Hemophilia Inherited? The hemophilia gene is recessive, meaning it is a characteristic that can be overruled by more dominant genes. Because females have two X chromosomes, the recessive hemophilia X chromosome usually is “overridden” by the unaffected dominant X chromosome. That is why females rarely get hemophilia. However, when the dominant X chromosome overrides the recessive hemophilia X chromosome, the female becomes a carrier of the hemophilia gene. Because males only have one X chromosome, if they inherit the recessive X chromosome, then they will have hemophilia because they do not have the unaffected dominant X chromosome to overrule the one that carries hemophilia. Although hemophilia is passed down from parent to child, it may not be seen for several generations. If mothers had female “carrier” babies or no male babies with hemophilia, then it can look like there is no history of hemophilia in that family. This can easily happen because the recessive hemophilia X chromosome is being passed on to the next generation by female carriers. One other way that hemophilia can occur is from a spontaneous genetic mutation or modification (change) in the cell that happens for no apparent reason. The mutation (affected X chromosome) can occur in the mother when she is conceived or when her child is conceived. Cells can mutate as they are created. It is estimated that cell mutations could account for about 30% of those diagnosed with hemophilia. Once the mutation is there, the gene can be carried through or passed down to future generations. A father with hemophilia will give his Y chromosome to all of his sons and not his X chromosome (which will come from the mother). The Y chromosome is what makes babies males, and the mutation that results in the factor deficiency is on the X chromosome. The Y chromosome does not have the hemophilia gene so the sons of a man with hemophilia will not have hemophilia (if the mother does not carry the hemophilia gene) and will not pass it on to his children. The hemophilia stops there. A father with hemophilia will give the affected X chromosome to all of his daughters. The X chromosome is what makes them female, and because the X has the hemophilia gene on it, all of his daughters will be carriers. What Is a Female Carrier? A female carrier is someone who has an affected X chromosome, which she can pass on to her children. Carriers can have normal levels of factor or low levels of factor. Some female carriers may have factor levels low enough to fall into the hemophilia range. These women may have bleeding symptoms and require treatment themselves. Factor VIII levels can increase during pregnancy, but Factor IX levels are fairly stable. We recommend that all carriers have their factor levels checked so they know their level. The female carrier has a one in four chance (with each pregnancy) of having a son with hemophilia. There is also a one in four chance (each pregnancy) of having a daughter who is a carrier. If the fetus is male, there is a 50% chance the boy will have hemophilia. If the fetus is female, there is a 50% chance the girl will be a carrier. These odds apply for each pregnancy a woman carries. So, a woman could have four sons and none of them have hemophilia, or she could have four sons and all of them could have hemophilia.
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