I have always enjoyed math for its absolutes. No matter what, two plus two always equals four. That is comforting somehow when in almost all other disciplines there are multiple interpretations of the same idea or theory.
You see, it is math then that can provide us with so much certainty in genetic genealogy. After all, even if you don’t know who they are, each of us has two biological parents. We have four biological grandparents, and we have eight great grandparents. However, the farther back we go the less we can rely on the merits of math. At the fourth or fifth generation back, what happens on paper is simply not reality. For example, on paper you should have 64 3rd great grandparents. However, many of us do not. For many of us that number is much smaller as the same person occupies more than one slot on our pedigree chart. While this significantly decreases the workload for traditional genealogy, it adversely impacts your ability to use genetic genealogy. Especially when it comes to that long long list of 4th cousins you have in your match list at any of the three major DNA testing companies.
Depending on how intermarried your lines are, you may be seeing individuals on your match list that genetically look like your fourth cousins, but they are genealogically your sixth cousins – EIGHT TIMES OVER! So how can you tell the difference?
There are two parts to that answer, one you can control, and the other you can’t.
While your fourth cousins and your eight-time-sixth cousins may look similar genetically, there are often small clues in the genetics that can help you tell the difference. This distinction can sometimes be detected by a testing company who, through research and validation, has been able to fine tune their algorithms to detect these subtle differences. You can participate in this double checking process by using some of the genetic tools that are available to you at Family Tree DNA, or at Gedmatch.com. But since most of you are not aspiring geneticists, let’s focus on the genealogical work you can do to determine if a match is truly a fourth cousin.
Before we jump in, remember that a fourth cousin designation just means that you and your match are separated by between six and twelve degrees, or people. So that might be five back on your chart to your common ancestor, and five down to your match, which would make you true fourth cousins, or it could be some other permutation of that.
For our example, let’s assume true fourth cousins. That means that the two of you share one of 32 3rd great grandparent couples. In order to find out which set, you have two genealogical identifiers: surname and location.
Therefore, the first thing you should do is make a list of the 32 surnames and locations of those couples (or, if you want to be an over achiever, make a list of all 64, which would include the maiden names of the ladies). Now, most of us do not know all 32 of those couples, so you are going to have some holes. Feel free to fill in those holes with surnames on subsequent generations that will carry through to this fifth generation. A great tool to plot your own list of ancestors is to use Google Earth. You can devote an entire folder to your 32 couples and easily plot your locations.
Then, begin to systematically evaluate the fifth generation of your fourth cousin matches for genealogical information that lines up with any of the items on your list. This is where Google Earth really comes in handy. You can even plot the surnames and locations of your matches. Google earth makes it very easy to see when your ancestral home may be bordering the locations of your matches. Those with whom you find a similarity become your best matches, and your best chance of determining your connection. Those without an obvious connection cycle to the bottom of your pile for a genetic evaluation, which we will cover next month.
You can perform these same kinds of searches for your second and third cousins as well.
As you begin to become more familiar with the fifth generations of your matches, you may also start to see patterns of surnames or locations emerge among your matches. These then become the surnames and locations that might be able to fill the missing spaces in your pedigree chart.
Originally Published at www.genealogygems.com