Most Recent Thoughts About a Most Recent Common Ancestor

I've done a lot of thinking about the potential for using DNA matches who share only a small segment in common with my target test-taker. At the first—almost exactly six years ago when my husband agreed to be my guinea pig and take the Y-DNA test as well as the autosomal DNA test at Family Tree DNA—I had heard warnings that any match sharing less than 100 centiMorgans would not be worth my attention.

"What?!?!" was my pained, shrieking reply to such an assertion. At that point, we didn't have any matches over that mark. As far as centiMorgan riches were concerned, our DNA test results proved we were poverty-stricken. Most of the matches I could account for were of people who turned out to be fourth cousins.

To further my research agony, looking at the pedigree charts for the closest of these matches was like reading through a nonsensical litany of foreign surnames. None seemed remotely familiar.

It took a while for me to gather my senses and realize something: any matches who turned out to be fourth cousins would need to have penciled in one essential element in their pedigree chart: the right third great-grandparent. If either I or my target match were missing a key third great-grandparent, we would have no trigger to alert us of the Most Recent Common Ancestor who would tie the two of us together.

With my current mission of (hopefully) breaking past one particular brick wall ancestor, Johanna Falvey Kelly, I am missing one key detail: the name of her parents. And yet, not only do I have clues regarding where in County Kerry, Ireland, she may have originated, but I have at least seven DNA matches to my husband—her second great-grandson, according to his pedigree chart—who also claim a Falvey heritage from County Kerry. Significantly, one of those is from New Zealand, neatly confirming Johanna's obituary report that she had a sibling who had emigrated to that very country.

The downside to this realization is that each of these matches shares far less than the once-preferred target level of 100 centiMorgans.

Yesterday, we sized up the level of each DNA segment shared by seven distant Falvey matches. Today, I want to look at this challenge from a different angle: what, exactly, we can expect from sharing a Most Recent Common Ancestor (MRCA) who is that distantly related.

There are two possible outcomes I am anticipating from these small DNA matches to my husband—each of whom, by the way, shares a Falvey entry in their respective pedigree charts. One outcome is that the connection would be through the line of a sibling of Johanna Falvey, which would mean the MRCA would be the parent of those two siblings—and thus, a third great-grandparent to my husband. The second scenario is that the connection would be through a cousin to Johanna, thus pushing the MRCA to the level of fourth great-grandparent.

In the first scenario, a likely relationship between my husband and his DNA match could be that of a fourth cousin. If we take a quick look at the interactive version of Blaine Bettinger's Shared cM Project at DNA Painter, we can see at a glance that fourth cousins could share anywhere from zero cMs to 139 cMs, with an average of 35 cMs in common.

In the second scenario, we would possibly be looking at fifth cousins, for whom anything from zero to 117 cMs could be shared, though generally it could be around 25 cMs as an average.

Of course, the candidates we've already identified as Falvey matches range from sharing 35 cMs down to only eight cMs. This agrees with the tentative level mentioned of fourth cousin for the higher end of the group of matches, though it is entirely possible that a wide range of other matches could be possible, like third cousins once removed. Especially with the long generations observed in my husband's family, that once removed, or twice removed, possibility needs to be kept in mind.

And yet, we also need to consider one other possibility: if any relationship beyond third cousin can share zero cMs, the amount held in common by any of our distant cousins could be an amount approaching zero, as well. Examining the "What Are The Odds" chart, assembled by Leah Larkin and available at DNA Painter, entering the value of "0" shows us the range of relationship possibilities for sharing absolutely no genetic material with a given distant cousin. Entering the amount of shared cMs for each of these seven Falvey matches gives me a range of possible relationships, along with the probability that each of them could, indeed, happen.

Admittedly, sliding down that slippery slope toward the twenty—or fifteen—centiMorgan amount often portrayed as the danger zone before entering the realm of Identical by State can be risky business. However, using a quantifiable chart demonstrating probabilities of specific relationship can be one tool to steady us on our mission past DNA landmines and towards documentable relationships by descent.