Family Tree DNA, 23andMe, and Ancestry DNA –all these companies will trace your origins based on genetic sequences from a DNA sample that you provide. Some will also venture to give you genetic results with implications for disease risk, which also implies risk for your children or for the child that’s developing in your right now. There are different arguments for choosing one service or another, depending on how much you want to know about your ancestors, whether you want to find long-lost relatives, or whether you want to help with a scientific project. In addition, certain companies are a better match for certain ethnic groups because of their particular database. In all cases, though, you should not get too worked up about the health-related results, one reason being that relevant genetic sequences do not necessarily mean that you will actually develop a specific disease.
What genetic family tracing companies test are sequences of a variety of genes that vary through the human population and between nationalities and ancestral groups. Some of these also happen to be relevant to disease, which makes sense if you consider that many diseases are particularly common in particular ethnicities. Tay-Sachs disease, for example, is more common in Ashkenazi Jews and French Canadians than in the general population. Sickle cell disease is most common in people with origins in certain regions where malaria has been rampant in recent centuries. Here, in North America, this means African Americans.
Tay-Sachs and sickle cell disease are textbook examples of recessive genetic conditions that will develop in a child if the child has a disease gene from each parent. Nevertheless, with sickle cell disease, there is some give and take on how severely the disease will manifest. Also, there are genetic diseases with dominant inheritance, where receiving one gene from just one parent, means that you will almost certainly develop the condition. In other words, the genes dominate, corresponding to a concept about genes that you may have been taught in high school biology, and that really dominated biology and medicine until fairly recently.
When a gene always exerts the effect that it is expected to have, this is called 100 percent penetrance. Tay-Sachs and sickle cell disease are two examples and there are many more, but with the majority of diseases –including heart disease, diabetes, cancers, and psychiatric conditions– the mere presence of genetic sequences associated with disease is not a guarantee that the disease will develop. There is not 100 percent penetrance. Instead, there is an interaction between genetic, environmental, and life-style risks.
This is something that has been observed for many decades, but it’s only in recent years that geneticists have been coming to understand. The interaction occurs at the genetic level and that’s where the science of epigenetics is concerned. Inside the embryo or fetus that’s growing within you are billions of cells, and every one of them has DNA containing all the genes needed to make a person. The cells that are developing into skin contain all of the genes, as do the cells developing into brain cells, muscle cells, bone cells, and all kinds of cells.
Why then does bone look so different from muscle? The answer is that genes can be turned on and off, like the apps in your smart phone. They are there, but you may not be using all of them. A muscle cell has all the genes turned off, except for the genes needed for what a muscle cell does. How active a gene is depends on several epigenetic processes, which continue to operate after a child has developed and throughout life.
These epigenetic processes that turn genes on and off are controlled by enzymes which themselves depend on genes, but they are susceptible to the environment, including factors that depend on life style. Smoking, for instance, causes epigenetic changes, thereby affecting which genes are tuned up or down in various cells throughout the body.
Epigenetic changes of genes are also passed down to future generations. Thus, if you smoke prior to becoming pregnant, your genes are modified in a pattern that’s passed down to your children. The same is true if your partner smoked before you got pregnant. Epigenetics works as much through fathers as it goes through mothers, and the changes can persist through a few generations. This means that some epigenetic characteristics of your genes trace back to your grandparents or great grandparents behaviors, even if the generations in between had a very different experience.
Does this mean that you should worry about having a great-grandparent who smoked? No, because the same epigenetic processes also respond to changes in a new environment, including your own. On the other hand, it could be significant if your grandparent was a starvation victim during the war, for example, because research suggests that such a physical and emotional stress could provoke epigenetic changes that increase the descendants’ risk for diabetes and obesity.
As for where we are headed with all of this, there are a few exciting areas. On one hand, researchers are looking at ways to take control of epigenetics with news drugs that could be used to treat and prevent disease. There is also an emerging idea that people should find out as much as they can about their ancestors, because our expanding epigenetic knowledge may soon make such information fairly important to understand one’s health and disease.