Thursday, July 9, 2020

A Different Way of Looking at the Odds

Ever since genealogists have been looking closely at how DNA can reveal the connection of familial matches, we've been equipping ourselves with charts. We have charts to tell us how likely it might be that any given DNA company would predict, say, our fourth cousin relationships with any accuracy. Likewise, we have charts to guide us in determining, given the amount of shared genetic material, just what that match-to-match relationship could be. Better yet, we've crowdsourced those numbers from a wide variety of research enthusiasts, and even updated those results and made their use interactive.

Those charts present a lot of numbers to use when struggling with a brick wall ancestor. They can tell me, for instance, that I am more likely to find a third or fourth cousin match at AncestryDNA than the other testing companies. Or that, given a specific centiMorgan count of shared genetic material, my husband's relationship to any other descendant of his second great-grandmother Johanna Falvey might be a third cousin once removed—but it could also be a third cousin twice removed, or a fourth cousin, or... or...

Just because, in the midst of this swirling overload of confusion, I need some encouragement, I decided to use a different way of looking at the odds, when it comes to comparing someone like my husband with his distant Falvey-linked DNA matches. Rather than bemoan the slim chances of correctly determining the nexus with his matches, let's look at how much we increase our chances of finding any answer when looking at such distant relationships as third cousin and beyond.

Granted, finding a missing half-sibling would be a sure thing, with a likely half-identical 1700 centiMorgans signaling the hit—but how many half-siblings are there out there, just whimsically deciding to give DNA testing a whirl?

If, on the other hand, we flip this scenario on its head, we see how much we increase our chances of finding a match on the far side of family connections. As the ISOGG Wiki noted in its section on "Cousin Statistics,"
Although there is only a low chance of sharing enough DNA with a specific distant cousin for the relationship to be detected, we have a large number of distant cousins.

The article provides a chart which outlines probable expected numbers of cousins, based on the degree of relationship—first cousins, second cousins, and beyond if that relationship can be detected through a DNA test. For instance, the chart shows an "expected number of detectable cousins" for first cousin relationships to be slightly higher than a count of seven. The number of possible detectable fourth cousins, on the other hand, could be well above four hundred. And that's a number of matches I'd much rather draw from, when figuring out missing family tree connections.

Genetic genealogy blogger Kitty Cooper put it this way: "How many cousins share my fifth [great] grandparents?" The potential number of those distant cousins can grow astronomically over the generations, as demonstrated by a simple chart put together by Canadian genealogy blogger John D. Reid. While those raw numbers may be cut almost in half by testing companies' inability to predict distant relationships at one hundred percent, that still leaves us with a larger universe of possible matches for any given umpteen-great great-grandparents.

While this is only a minute dynamic in the universe of cousin-matching efforts, it is the silver lining that encourages us that the likelihood of finding a match may be at those outer fringes of relationship. The only catch is that the weight of the proof must shift to the reliability of the documentation assembled to confirm just how two matches are connected.

Without that Falvey connection in those matches' trees, my attempt at determining Johanna Falvey's family constellation might be near as impossible as, say, for an adoptee seeking birth parents through distant cousin relationships. There does need to be a secondary guide post—like a shared ancestral surname—to keep us on the right path while we reach back to distant connections. But the fact that, in our collection of DNA matches, we are more likely to encounter the key to a second great-grandparent than anyone closer is enough to keep me seeking answers.

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