Doing the Math on DNA

After having sniveled and complained about my research progress in yesterday’s post, with the receipt of one email, I may have to change my tune.

Just before writing my statistical report for yesterday, I had scrolled through my DNA test results, double checking those numbers I was about to report. Among my most recent matches on the autosomal DNA test was a person who showed up in the predicted range of second to fourth cousin.

While that may seem like a distant relationship to most people, you and I and anyone dabbling in genealogy will know that we are far too well equipped to be fazed by a label like “fourth cousin.” We’ve done our due diligence and we are prepared!

When I replied to that potential match, I realized her email address was exactly the same as another candidate I had just received in that same range. What was going on here? Program malfunction?

No, as it turned out, one person was serving as administrator for DNA test results for two people. Not unusual—I do the same for my brother and my husband. As my husband is so fond of saying, he doesn’t “do” genealogy—he just carries the bags.

I wrote this two-for-one party at the other end of the duplicated email address, and in less than twenty four hours, I had a reply. Not only was this person doing double duty, but she was actually shepherding four DNA tests through their paces. And I was a match to every one of them.

As has so often happened in this project, the other party was stumped over how we might connect. This is head banging, hair pulling frustration. I cannot tell you how many times I go through this exact scenario: we share trees, we look through each other’s data, and not one mutual surname can be found. The only encouraging aspect of the routine is that I usually undergo this reaction when trying to match up with people predicted to be at the range of fourth cousin—or worse. This time, I had a chance at connecting at a mere second cousin ranking.

How hard would that be?

So I rev up my old harangue about how calculating the connection between two second cousins would require finding a mutual relative at the level of great-grandparent. Remember, the most recent common ancestor for first cousins would be a grandparent. At the second cousin level, it would be first great grandparent. For every increase in level of cousin, you can minus that count by one and tack it onto the label of great grandparent. In other words, n cousin equals n – 1 great grandparent. See? Simple math.

It doesn’t take a rocket scientist to inform us that, at the outside chance that this new match and I are related at the farthest level—fourth cousins—this worst case scenario would require us to identify a most recent common ancestor at the level of third great grandparent to confirm the proposed match. Of course, only a devoted genealogist would be prepared to serve up information like that. But we are up for this game—at least, if it involves my maternal line.

If, on the other hand, the connection is on my paternal side, well…you’ve already seen how little research I’ve been able to accomplish on those camouflaged Polish immigrants.

Not to worry, though: I have a handy trick in my back pocket. By comparing any unknown person’s results with a relative I’ve already tested on my father’s side, I can eliminate any matches linked to that side by using the “not in common with” function at the Family Tree DNA website. With one click of that button, I magically remove from my list anyone whose DNA also matches the near relative on my paternal side—well, all twenty two of them.

Small numbers which work hard can still be our friends, however. And when I put my data through their paces in that manner, I see those four related matches from my new source still remain. I can safely assume they connect somewhere on my maternal side.

Granted, that side holds the preponderance of my test matches. But if I can isolate even one additional match and confirm that it belongs to a specific surname on my maternal side, I can play that reduction game once again: find out who is in common with that other party. Surname by surname, I can eliminate those lines among the sixteen third great grandparent candidates on my maternal side that don’t connect with these four related matches. Yes, I’m chipping away at this monolith with a toothbrush, sweeping away the residue, but eventually I’ll be left with some rock-solid results.

Once again, numbers are not only my friend, but they show me just how far I have to go and how long I have to hold on before I hit pay dirt. You can’t pace yourself without measurements.