Family DNA is more technically known as autosomal DNA. It is DNA that can be measured in both men and women, and it provides us a bit of the ancestral story from both parents of each person tested.
Every person has 22 chromosomes from each parent, plus the X chromosome from their mother. (Men also receive a Y Chromosome that is not part of Family DNA) So, half of our DNA comes from out father, and half from out mother, excluding the X and Y chromosomes. In a perfect world, the half of the DNA we receive from our father would also be exactly half from his father and half from his mother. The same would be true about what we receive from out mother. We don’t live, of course, in a perfect world.
As it turns out, our grandparents don’t always share equally in the amount of DNA we get from them. One grandfather might provide a bigger chunk than his spouse, while it might be quite different percentages from the other set of grandparents. That means that the grandparent whose DNA we are missing and all of his or her ancestors will be absent from that time forward.
There are several implications. First, it tells us that just because we don’t have DNA that appears to match a set of great grandparents we thought were ours, doesn’t mean we aren’t related to them. We may just not have received a big enough chunk of their DNA. Second, it tells us that as we seek to find matching cousins, we will have some lines where we seem to have lots of matches and other lines where we don’t have as much. At the same time, our sibling may have gotten a different dose that will give them a different set of matches from us. Third, we learn that finding matches will be easiest in the first few generations but harder with every passing generation, except for the occasional DNA hogging family line that keeps contributing more than its share.
We just need to keep in mind that if shared equally, we would have half of our DNA from out mother, 1/4th from each of her parents, 1/8th from her grandparents, 1/16th from her great grandparents, and so on. The same would be true for our father. By the time we get to our own three-great grandparents, we would only have about 3% of our DNA from each. It is likely that in some cases, we would have even less because of the way DNA is passed along.
Even with that, Family DNA can be very helpful in uncovering our family story. However, it requires significant research and a precision that goes beyond just finding people who match us and assuming that their match comes from a particular family line since their ancestors lived in the same area as our own.
When we are working with Family DNA and trying to match people more than five or six generations back, we are most often dealing with levels of probability rather than absolute proof. When we put it together with the conventional genealogical research we have done, we are able to increase the likelihood of those probabilities. Still, we must do our best to be open to being surprised by finding family lines that we didn’t know existed.
We have a couple of examples of Family DNA work we have been doing. These examples point us to what appears to be true, and in both cases, the DNA seems to confirm what the paper trail was suggesting. We will keep piling up the evidence as more people take tests through such organizations as FamilyTreeDNA.com and Ancestry.com., among others.