Sometimes, a step backwards can get us moving forward.
After working on my mother-in-law's Ijams and Plummer line for half a month, I thought I'd check on the most distant ThruLines report for that family line at Ancestry's DNA to see if there were any updates. There were—well, there were, if you count a diminishing number of results as progress.
In the past, Ancestry's ThruLines had shown five or six children descending from William Ijams, grandson of Elizabeth Plummer Ijams, and fifth great-grandfather of my husband, who is the surrogate tester standing in for my mother-in-law. Today, however, there were only two descending lines, and each of them, thankfully, can be confirmed through documentation.
Since connections with fifth great-grandparents is as far back as ThruLines shows for autosomal DNA tests, William himself would have to stand in as proxy for his paternal grandparents' DNA composition—the best I could do under these testing conditions.
Today, however, those stray other lines—names listed in previous ThruLines results that I hadn't been able to confirm through documentation—have simply vanished. Poof! If the DNA test candidates represented by those ThruLines results were indeed distant cousins, they obviously must have been connected through a different genetic route. Perhaps, someone had presumed there was a connection and had made a mistaken entry in their own tree which, repeated as others copied that tree, got picked up by ThruLines.
Though it is theoretically possible to find DNA matches who share a most "recent" common ancestor at a level of seventh great-grandmother, as Elizabeth Plummer Ijams would have been to my husband, it is not likely to confirm such a match. On average, DNA matches who are eighth cousins, as such a descent from seventh great-grandmother would yield, would share 0.000763% of their genetic makeup, according to a chart drawn up by Hope Carnicle, reported by a post on the ISOGG.org wiki.
In other words, eighth cousins could share up to forty two centiMorgans. Or they could share none.
In most cases, we'd never see such DNA matches, because the odds are against us. In my mother-in-law's case, a second strike would come in the form of multiple intermarriages over those many generations spanning her family's heritage, so even if a segment match registered, we'd have to delve deeper to determine which ancestor actually contributed that match. It might not be the ancestor we were suspecting.
In the end, while this change in results at Ancestry's ThruLines report doesn't strictly lead us to matches who share Elizabeth Plummer's DNA, it does zero in on those matches who actually were descendants of Elizabeth's grandson William Ijams. A far more accurate report may do nothing more than bolster my confidence in the tool, but a gesture like that can go a long way, in my opinion.

