Thursday, June 2, 2022

A Far - Reaching
Most Recent Common Ancestor


Introducing the use of DNA into family history pursuits interjects new-to-us terms such as "Most Recent Common Ancestor." When I first heard that term's acronym—MRCA—bandied about, I thought someone was talking about a new pandemic. Not so: the new quest, in genetic genealogy, is to find the closest ancestor held in common between a DNA test-taker and his matches on the same family line. That's what finding the MRCA is all about.

In my husband's case for this month, the work of finding that Most Recent Common Ancestor has already been done. He is John Gordon, born somewhere—the jury is still out on whether he originated in Scotland or Germany, two very different options—about 1739. That, at least, is the date given on his memorial erected at the Gordon Cemetery in Greene County, Pennsylvania.

John Gordon was my husband's fifth great-grandfather. And that's the sticking point I'm musing over today. When most people take a DNA test for genealogy purposes, they likely hope to find cousins somewhat closer than fourth cousins—certainly not sixth cousins or beyond. Working with connections of that closer sort, a MRCA might be, say, a second great-grandparent, a far more easily researched proposition.

Working with the present-day descendants of a fifth great-grandfather, however, means not only that we and our DNA-matching cousins have done the research work to push the generational boundaries of our pedigree chart out sufficiently far. It also assumes there will be enough genetic material to share between sixth cousins. In many cases, that possibility approaches zero. Depending on the testing company used by the two potential matches, sixth cousins may find each other about eleven percent of the time, if testing at AncestryDNA, or below two percent of the time, if using Family Tree DNA. That's quite a spread of possibilities.

Put another way, the likelihood of sharing no detectable DNA with a sixth cousin approaches ninety percent, according to a research team at Cambridge University.

By now, you are getting my point about such distant MRCAs. So, how do we proceed in letting DNA guide us through our spring-cleaning research task this month with the line of John Gordon?

Incredibly, it appears that fifth great-grandfather John Gordon must have had some persistent genes. Of course, a little pedigree collapse doesn't hurt the equation, either. The Gordon line is one in which various members of what was once a very large family have intermarried over the generations. In my mother-in-law's case, that meant two of her great-grandparents—one on her maternal side, the other on her paternal side—were both descendants of that same John Gordon.

The end result, at least as far as my husband's sixth cousin DNA matches are concerned, is that the highest connection is a match who shares thirty centiMorgans with him—all within one single segment. If you're wondering what the odds are of that happening, it turns out there is indeed a three percent relationship probability for such a scenario. A stretch, indeed.

So, despite such a far-removed MRCA as we've found in John Gordon, it looks like I'd better get busy comparing notes on those Gordon DNA matches. As it turns out, there are no less than 165 such matches, according to Ancestry's ThruLines® readout. John Gordon's reach has indeed gone far and wide.


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