Thursday, February 23, 2023

On the Precipice of Possibilities


One helpful tool which has been borrowed for genealogical purposes has been that of DNA testing. Finding matches with distant cousins who have taken autosomal DNA tests can be informative, helping us fill in some of the blanks on our family tree. But when we get to the point of examining fellow descendants of our fifth great-grandparents, we begin to peer over the precipice of possibilities.

Think of it this way: any DNA matches I receive who also claim my fifth great-grandfather Zachariah Taliaferro as their ancestor could be related as remotely as sixth cousin—or farther. The trouble with such DNA analysis is that we share less and less of an ancestor's genetic signature with each generation we are removed from that individual.

According to a chart featured in the ISOGG wiki (International Society of Genetic Genealogy), based on the particular DNA test I'm using—at AncestryDNA—the probability that two fourth cousins will share enough DNA for the relationship to be detected by testing is seventy one percent. For fifth cousin relationships, the percentage drops to thirty two percent. And sixth cousins? My chances drop to eleven percent that another descendant of Zachariah will match me if we both test at that same company.

All that to say there are a lot of Taliaferro descendants who will never show up on my list of DNA matches at

Still, I'm curious, so I popped over to Ancestry's ThruLines readout to see what I could find. I wasn't particularly overwhelmed to learn that I may currently have up to eighty seven matches who also claim the same Zachariah Taliaferro as their ancestor. After all, we're talking enough generations to get us back to a fifth great-grandfather. 

The key, in my mind, was to see how many centiMorgans any given match shared with me. Matches sharing less genetic material could actually be a coincidental connection, or what is sometimes called identical by state, rather than identical by descent. In this particular case, my ThruLines Taliaferro candidates share anywhere from a high of sixty two centiMorgans down to the lowest at six.

Naturally, the advice would be to disregard those "matches" falling at the lower end of the range (I've heard that advice for twenty cMs or less). But what if the "match" can also genealogically support what the test seems to be indicating about a genetic family tree?

Right now, I'm working through that puzzle. I found it interesting that, of all Zachariah's children, those of the son who is also my direct line ancestor—also named Zachariah—add up to forty five of those eighty seven Taliaferro DNA matches. My next largest set of Taliaferro matches, twenty nine in total, belongs to the descendants of Warren Taliaferro—no surprise here, since I also descend from one of his daughters. The remaining thirteen matches are scattered among four other children of Zachariah and Mary Boutwell Taliaferro: Benjamin, Frances, Mary, and Sallie.

At first glance, I could see that some of the trees offered up by the ThruLines process don't seem to be verifiable by documentation. On the other hand, the challenge here is to locate those hard-to-find records from the late 1700s and early 1800s to confirm the lines of descent. When that obstacle is combined with a low centiMorgan count, I tend to disregard the "match" and move on to a more reasonable proposition.

Working through this process step by step may seem tedious, but it is one way to organize my search for verification of lines of descent. While I am tottering on the edge of genetic relationship with those lower centiMorgan counts, I'm keeping in mind the likelihood that moving backwards another generation will be taking a step over the edge of what can be verified by DNA tests—other than by Y-DNA (for any Taliaferro men willing to participate in such a project) or by mtDNA, as I have done to connect through the matriline of Warren Taliaferro's wife.

Even though we'll now be stepping beyond the reach of autosomal DNA tests, I want to learn about the ancestral lines preceding Zachariah Taliaferro. We'll begin tomorrow to review what can be found on the next generation.   

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