Sunday, October 22, 2023

The Complications of Pedigree Collapse


When constructing a family tree, perhaps we expect a diagram with lines stretching straight back into the past, never giving a thought to the possibility of tangled lines crossing and recrossing each other. However, when a family settles in a limiting area—whether the proverbial desert island or a small valley enclosed by impassible mountains—their children's choice of mates becomes limited, as well.

I never thought of southwest Virginia or the neighboring land in northeastern Tennessee as being land-locked in any limiting way. Apparently, when searching for a prospective spouse, some of my forebears may not have looked as far away as they could have done, for one DNA match found on my Tilson ThruLines results pulled me up short this weekend. I totally forgot about the possibility of pedigree collapse in this longstanding family settlement.

Working my way through the matches sharing my fourth great-grandfather, Peleg Tilson, I had just advanced to another of his daughters, someone with the rather common given name of Mary. To make matters worse, Mary happened to marry someone with a name designed to create an even more challenging research prospect: John Brown.

I didn't have much detail on this couple, other than the information I had gleaned from an old Tilson genealogy published in 1911, but supposedly, the first DNA match I had for this ThruLines entry shared far more genetic material with me than I had come to expect.

Most of the matches connected to me through Peleg Tilson's descendants are at least my fifth cousins, sometimes even fifth cousins once removed. If a cousin at that distant level of relationship shows up in my matches at all, the centiMorgan count is generally very low. True, according to Blaine Bettinger's Shared centiMorgan Project at DNA Painter, a fifth cousin once removed could share up to eighty centiMorgans with me—but could also approach zero. An expected average is likely to be around twenty one centiMorgans, but in many cases of my Tilson cousins, the numbers dwindle precipitously lower than most people consider reliable.

Contrary to those minuscule matches, though, the DNA match with the highest count under Peleg's daughter Mary came in at fifty eight cMs, which surprised me with its stark difference. Taking a closer look, I realized this match was already tagged in my records as a match—for a different ancestor. That other ancestor came from my maternal grandmother's Broyles side of the family, while the Tilson line reaches down to her husband's line.

While I already knew this DNA cousin was connected through my grandmother's Broyles aunt, in seeing this result, I recalled that the aunt had married someone who came from a family with a long history of calling that corner of Tennessee home. With yet another unhelpful name to research—this family was named Jones—rather than taking the usual approach to checking ThruLines results from the ancient ancestor forward in time to the current generation, I worked my way backwards on the ThruLines results. I wanted to see just how this Broyles cousin was connected to my Tilson line.

Thus, I worked my way through documentation from this Jones cousin, backwards in time until the Jones surname gave way to that other research challenge, the Brown surname. Along the way, I spotted collateral lines with names too familiar to be coincidental name twins, and my head ached with thoughts of how much this tree may call for cleaning up duplicate entries. Cousins marrying cousins and other near misses make for many déjà vu moments in family history. Time to warm up that "merge with duplicate" button on my family tree.


  1. Every now and then I am baffled by a high cM count on someone who looks like a 5th or 6th cousin. That's a good description of possible reasons and research complications. And like you, I usually think of Tennessee and Kentucky to be historically flush with families passing through in all directions. While that would seem to offer plenty of unrelated marriage prospects, I guess it also was a meeting place for people whose families originated in nearly the same places.


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