If I have the genes for Factor 5 Leiden, do my siblings and children?

Factor 5 is a protein that is important for forming blood clots.

People usually have two copies of the Factor 5 gene. Sometimes the Factor 5 gene has a change in it called "Factor 5 Leiden." This change causes the protein to be overactive. An overactive Factor 5 protein increases the risk of forming blood clots too easily.

A person with one normal Factor 5 gene and one Factor 5 Leiden gene (we call this combination "heterozygous") has only a slight increased risk of a blood clot. A person with two Factor 5 Leiden genes (this combo is called "homozygous") has a much higher risk of clotting.

You asked about a person who has both genes for Factor 5 Leiden. In most cases, the mother and father of this person will each have one normal gene and one Factor 5 Leiden gene.

The brothers or sisters of this person might have:

• Two copies of the Factor 5 Leiden gene (a 25% chance)
• One copy like the parents (a 50% chance)
• Two normal Factor 5 genes (a 25% chance)

You also asked about the children of a parent who has two copies of Factor 5 Leiden. Each child has a 100% chance of having at least one copy of Factor 5 Leiden. The chances are even higher if the other parent carries a Factor 5 Leiden gene. In that case, a child could get one copy of Factor 5 Leiden (50% chance) or two copies of Factor 5 Leiden (50% chance).

This gives you some idea of the risks to family members. But the only way to know if each person has a risk is for them to be tested for the Factor 5 Leiden gene. Having one copy of the Factor 5 Leiden gene is not a big risk, so it makes sense to test only the people who are at risk of having two copies. To help you understand who should get tested, talk to a medical professional who is familiar with genetic testing, such as a genetic counselor or medical geneticist.