Of course we know that all human beings are related, but sometimes genetic analysis drives this point home even more strongly. If you carry a rare variant in the Factor V Leiden gene – which has just been added to the Navigenics genetic testing service – you share a common ancestor with every single person who carries that variant.
Factor V (pronounced “factor five”) is a “founder mutation,” a genetic change that originated in one individual long ago and can now be found in that person’s descendants around the world. This mutation is present in two to 15 percent of Caucasians, but is rare in genetically distinct non-Caucasian populations.
It is thought that this mutation arose in a single Caucasian person after Caucasians diverged from Africans and Asians thousands of years ago. The mutation disables the body’s ability to dissolve blood clots, and can lead to serious clotting-related health problems such as deep vein thrombosis (DVT) and pulmonary embolism, a blood clot in the lungs.
You’d think that with such a serious potential health issue, the Factor V “founder” and his or her descendants wouldn’t have lasted long. But founder mutations aren’t that straightforward.
Some founder mutations, for example, survive in populations because they are relatively harmless. One good example of this is the gene variant that causes lactose intolerance. If you have two copies of the risky gene variant, you probably get stomachaches after drinking milk, but that shouldn’t get in the way of your surviving and reproducing!
But why would the factor V mutation, which can cause serious illness and even death, continue to survive and be passed on in a population? Scientists aren’t sure yet.
Sometimes, a serious mutation survives in populations because it is actually beneficial to carry one copy. These mutations only become harmful when two people who carry the mutation each pass it on to their child, who then has two copies. One well-known example of this kind of founder mutation is sickle cell anemia. This disease is most common in people from parts of Africa and the Middle East, regions that also have high rates of malaria.
People with two copies of the sickle cell gene have red blood cells shaped like a sickle, or “C”, and generally become very ill in childhood and have a shortened life expectancy. However, people with only one copy of the gene variant are protected against malaria infection, because of the unusual shape of their red blood cells. This selective advantage allows the gene variant that causes a serious, potentially fatal disease to continue to be passed on in the population.
It is possible that the Factor V mutation also confers a selective advantage to people who carry one copy, but scientists just aren’t sure yet.
If you carry the mutation, you can take steps to prevent becoming ill as a result. And, if you meet someone else who carries the mutation too, it’ll be a little like finding a long-lost brother or sister! Somewhere back in the human genetic family tree, the two of you have a “Factor V” ancestor in common.