Tuesday, July 7, 2020

Why a Small Link Can
Make a Big Difference

Many of us who have learned how to use genetic genealogy to address the mysterious gaps in our family tree have been taught to shy away from matches sharing smaller centiMorgan counts. Much of what gives us our matches when we purchase a DNA test is a mathematical calculation of the likelihood that any two cousins, say, would share the same genetic patterns. The closer the cousin relationship, the higher the measurement—in what is called centiMorgans—of matching genetic material.

Some companies are apparently better at calculating the level of what we might call cousinhood, as we can see from this chart provided by the International Society of Genetic Genealogy. Closer relationships have a better chance of being labeled accurately, though relationships beyond third cousin can sometimes be accurately predicted—though with varying degrees—depending on the testing company.

This, however, is not necessarily why smaller centiMorgan levels usher us into a danger zone of prediction reliability. The ISOGG chart, for instance, can help us infer how many cousins we won't match by DNA who, in reality, do descend from the same common ancestor as we do.

The flip side, though, is discovering someone who does share a sequence of genetic material with us who could turn out not to be an actual relative. For that differentiation of results, science offers two terms: Identical by State versus Identical by Descent.

Identical by Descent—or IBD—is what we are looking for, of course, but what we may hear from some instructors in genetic genealogy could be a specific number past which genetic matches would be merely a matter of coincidence. "Yep, he's Irish, just like me" would be the response to discovering a shared genetic segment received not by virtue of the paper trail of our pedigree, but because the match is merely Identical by State—or IBS.

Crossing that line from the one (IBD) to the other (IBS) has caused some to offer, in their professional opinion, a target number to avoid. In my years of attending genetic genealogy trainings, I have heard everything from "don't go under 100 centiMorgans" to "don't go under ten." Many have solid reasons why twenty cMs is their avoidance zone of choice.

So why, in my current project of examining the family history of my husband's second great-grandmother Johanna Falvey Kelly, would I risk going below that jinxed twenty centiMorgans? This is where I need to have a discussion with myself and think aloud about research strategies.

While the risk of an IBS situation comes from a small segment "shared by many people both within and between populations...which have no genealogical revelance," another aspect of that small segment may simply be that it is of a size signifying relationships beyond the reach of a paper trail. The ISOGG Wiki, for instance, offers the danger zone as shared matches of fifteen centiMorgans or less:
Many matches under 15 cMs will in any case share ancestry more than ten generations ago and will be mostly beyond the reach of genealogical records.

However, if, in my strategy, I pull up all my husband's DNA matches at Ancestry, and then search those results to select only those matches who also contain a Falvey surname in their pedigree, wouldn't the coupling of a known surname with that low centiMorgan count mitigate the problem somewhat?

So far, at least among the pertinent AncestryDNA matches, I have seven possibilities to work with. Of those, the largest match shares one segment at 35 cMs with my husband, and the smallest match contains one segment of only eight cMs. Every one of those matches contains a Falvey in their tree from County Kerry, Ireland—though I have great doubt regarding the accuracy of a few of their choices for supporting records.

Of those seven matches, I've made a chart listing the names of the "founding" ancestor, being the couple born in County Kerry. Only two trees seem to include the same Falvey couple, and a third tree matches relatives from a solid match gleaned from a different testing service. Some of those County Kerry Falvey ancestors left for New Zealand, which is an encouraging sign regarding Johanna's story, as she had at least one sibling who moved to New Zealand. Others moved to the United States, settling in Detroit, or Connecticut, or Massachusetts, or Rhode Island. And then, of course, there was Johanna, moving to Fort Wayne, Indiana.

I'll continue matching information from the trees of each of these seven Falvey connections, looking mostly for documentation to signal we've got the right path back through the generations. I'll also take the surnames of the earliest Falvey ancestors in each tree to see whether I can find them in the baptismal and marriage records in the townlands where I believe Johanna originated.

If all of these details can line up, regardless of the limit of the small centiMorgan count shared, the result may be that these tiny clues could indeed lead me to the right verifying records, and thus confirmation of connections with an earlier generation—the goal of my research quest.

This is not the first time use of a small segment match has led me to break through a genealogical brick wall. It is beginning to show me that, with due caution, combining these small genetic segment matches with specific surnames can lead to confirming documentation which contained the answers I was looking for, all along. Of course, the document was always there; it was finding a way to confirm it was the right document that I've needed.

While I'll have to step carefully through the research landmines that can surround me, holding closely to those guideposts should help me discern which ten- to fifteen-centiMorgan match does, indeed, confirm it is Identical by Descent.


  1. You're on the right track. I have a 3c1r on paper. I knew her before we did DNA. We have a 7cm match. So the threshold on Blaine's chart is 0. It made sense to me!

    1. So true, Kat. Blaine re-emphasized that, about the zero cMs shared, in his presentation at the most recent NGS conference.

      Like you, I have since spotted, among my DNA matches with the tiniest of cMs shared, the name of a distant cousin I already knew was documented on my Ancestry.com family tree. It does make sense--but while those small cM shares are possible, we need to try and anchor those possibilities with other guideposts, like a reliable paper trail or personal family knowledge. In a way, those known details provide a different form of triangulation.

  2. This gave me some good food for thought. I haven't really examined any of those smaller matches yet. I'm still hoping for that elusive sibling match, but have nearly given up hope.

    1. Oh, please keep going, Sara! I know it may seem frustrating, but sometimes, those smaller matches do lead to a closer relative.


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