Friday, July 28, 2017
DNA Testing :
Why We Can't Shoot for the Moon . . . Yet
While I may have made some astute guesses in figuring out who the parents were for my orphaned second great grandmother, Mary Rainey Broyles, I'm always hoping for further confirmation. A researcher is seldom satisfied.
It was indeed gratifying to find an exact mitochondrial DNA match linking me to the presumed third great grandmother to Mary—a woman named Margaret Watts, wife of William Strother of colonial Virginia. It also bolstered my confidence to see among my autosomal matches a fourth cousin descending from Mary's (erstwhile presumed) brother Thomas Rainey.
But I'm always wanting more confirmation. So why not look for a DNA match descending from an ancestor preceding that brick wall of the orphan's parents? After all, couldn't finding a descendant of Mary's presumed parents or grandparents cement my case even further?
At this point in the state of the art, I'd say that is the genetic genealogy equivalent of shooting for the moon. Perhaps someday; not quite yet.
Here's why: currently, our DNA test results are based on samplings of specific parts of the human genome; it doesn't include every bit of the data. When a whole genome test becomes available to the general public at a reasonable cost, things will be different. But for now, only specific sections of the genome are sampled. We aren't getting the full story yet.
There's even more to consider, as was capably pointed out by The Genetic Genealogist, Blaine Bettinger, when he stated in a 2009 post that "everyone has two family trees: a genealogical tree and a genetic tree."
In examining the possibility of genetic material shared by third cousins from their most recent common ancestor, Blaine observed that it is possible these two distant cousins "have segments of DNA from these ancestors, but they wouldn't show up as a match...unless they [possessed] the same segment of DNA."
So, first we need to remember we can compare segments only because they are part of the sampling included in the DNA test we are using. Then we need to realize that while we do receive small portions of DNA from these more distant common ancestors, there is nothing guaranteeing that all third cousins (or fourth cousins or beyond) will receive the exact same smidgeon from that exact same common ancestor.
That's where Bettinger's "genetic tree" comes in. Your genetic tree illustrates the DNA segments inherited from specific ancestors. Only at the nexus of the right segments in the right places—both your specific chromosome and your cousin's—will you have a DNA match. As Blaine observed in answer to a reader's question, a person may be third cousins with another relative on paper, but that doesn't necessarily mean they will both show up as DNA matches.
If that is so for a match between third cousins, then it is surely even more so for fourth cousins—which is the relationship I have with the Thomas Rainey descendant I match.
It would be even less likely if I were to hope for a match among those descended from the next generation up. In other words, while I am fortunate to have found a fourth cousin match to help muddle through this orphan puzzle concerning my second great grandmother via her brother, it would be even harder to ascertain her parents by seeking autosomal DNA matches from that more distant generation. Confirming relationship to Mary Meriwether Gilmer and her husband Warren Taliaferro as most recent common ancestor would put us in search of fifth cousins—doable, but not necessarily guaranteed to confirm.
On the other hand, if it happens, it happens. Any DNA match, obviously, yields the story of shared genetic material—as long as we can confirm through documentation which way the genealogical pathway went. But to go looking for fifth cousins on paper, then ask them to consider DNA testing, does not necessarily guarantee DNA confirmation of the relationship. Not at that distance.
Just as we have far to go before the general public can access full genome testing for a reasonable fee, the genealogical world has far to go before we can say we have access to every digitized document which could confirm our paper trails. Targeted research in locales pertinent to my ancestors' residence and migratory ways may yield a more readily accessible confirmation of my guesses about my second great grandmother's parents than can be had from the next generation of DNA tests at this point.