Thrombophilia affects at least 19 million people in the U.S. Yet, many people who have thrombophilia do not know it.
A person's family history plays a significant role in determining who gets thrombophilia. Thrombophilia is inherited in a dominant pattern, which means that close relatives (parent, siblings, or children) of a person with the altered gene have a 50 percent chance of also having the altered gene. The more distant the relationship, the less likely it is that a person has inherited the gene.
| Relationship to Person with Thrombophilia | Risk of Inheritance |
| Parent, siblings, and children | 50% (1 in 2) |
| Grandparents, aunts, uncles, nieces, nephews, and half-siblings | 25% (1 in 4) |
| Cousins | 12.5% (1 in 8) |
Inherited thrombophilia also occurs more frequently in certain ethnic populations.
| Ethnic Group | Factor V Leiden | Prothrombin |
| Caucasians (Northern European) | 5 - 7% | 2 - 3% |
| African Americans | 2.2% | 0.2% |
| Hispanic Americans | 2.2% | No data available |
| Native Americans | 1.2% | No data available |
| Asian Americans | 0.45% | No data available |
Most people with thrombophilia have only one copy of a genetic alteration; they are called heterozygotes. A small number of people have two copies; they are called homozygotes. Since factor V Leiden and the prothrombin mutation are relatively common in some groups, some people inherit one copy of each genetic alteration — one factor V Leiden and one prothrombin mutation. They are referred to as double heterozygotes.
Having more than one inherited thrombophilia mutation greatly increases the risk of developing a potentially dangerous blood clot. For factor V Leiden homozygotes, the risk of having a blood clot is 50 to 80 times higher than the average person's risk. A person who is a double heterozygote has a risk that is 20 times higher. Similarly, prothrombin homozygotes also have an increased risk, but the exact amount is not known.
