How is hemophilia inherited?
Each individual’s information is saved in the DNA and organized in genes, which form separate segments in the DNA chain. Each gene contains the data of a particular characteristic. At the same time, chromosomes are made up of genes. Humans have a total of 46 chromosomes (23 pairs): half come from the mother and the other half, from the father. One of those pairs is the sex chromosome, which determines whether a person’s sex is male or female. Females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY).
In order to describe how hemophilia is passed from parent to child, we will see some examples that will be very helpful. However, it is important to understand that the following examples are based on probability and are by no means an exact calculation because they depend, among other factors, on each person’s particular attributes.
The sons of a man with hemophilia and a woman that does not carry the bleeding disorder will not have hemophilia, while the daughters will be carriers.
A man that does not have hemophilia and a woman that carries the bleeding disorder have a 50 % chance of having sons and daughters without hemophilia, a 25 % chance of having female carriers and a 25 % chance of having sons with hemophilia.
The only chance of a woman with hemophilia being born is the union between a man with hemophilia and a female carrier.
There are other bleeding disorders that can be inherited. The most common one is the von Willebrand disease (VWD), which is caused when the molecule that protects factor VIII in the bloodstream fails. The platelets do not attach properly to the vascular damage after the injury and the factor’s efficiency decreases. People with VWD usually encounter less complications that people with hemophilia. It is inherited in a different way and men and women are equally affected. If one of the parents carries the disease, they will have a 50 % chance of having a son or a daughter with VWD.
Finally, alternative ways of acquiring hemophilia exist. A spontaneous mutation of the gene of the chromosome X can happen without a family history of hemophilia. And it also can be acquired, for example, by a woman in the postpartum period. In this case, the clotting factor deficiency is caused by another illness.