Thursday, May 26, 2016

On the Eve of Another Change

No sooner had Ancestry DNA announced that they had made plans to adjust their algorithm for matchesand created a furor over impending changes in the processthan Family Tree DNA came out with an announcement of their own.

I first found out about it when making the rounds of all my favorite bloggers, especially the ones I can count on to keep me up to date on genetic genealogy news. First announcement I ran across was from John Reid's Canada's Anglo-Celtic Connections, where his article included a link to Blaine Bettinger's blog, The Genetic Genealogist.

In explaining that FTDNA intends to update its matching thresholds, Blaine included a helpful flow chart in his post. In addition, he, too, linked to the other major bloggers covering genetic genealogy, including Debbie Kennett, Roberta Estes, and Judy Russell. While some of these posts added little to what Blaine had already covered, there were a couple helpful observations.

In Debbie Kennett's blog, Cruwys News, she noted that the challenge with setting a cut-off limit is a walk down the fine line between false positives and false negatives. Because FTDNA doesn't phase their data (differentiate between chromosomes originating from the paternal side versus the maternal side), smaller segments do run the risk of yielding false results.

On the other hand, she explained another point to remember:
if you match on a single segment under 10 centiMorgans you will not share a common ancestor within the last ten generations. Even matches of 10 centiMorgans can be very distant.

With this proposed revision in mindand FTDNA's anticipated outcome slated to yield "only minor changes in their matches"I took a look at just how many of my own distant matches fall under that 9 cM threshold.

Out of nearly 1,160 matches in my autosomal results at FTDNAa number incrementally growing every weekit turns out that from match number 811 onward, the "longest block" slips below the 9.00 cut-off point. Of course, there is a further option with the second node in FTDNA's new decision treein which single segments falling below 9 cMs may still be considered if the total amount of shared DNA is greater than 20 cMsbut I'll set aside that thumbnail sketch for another crisis.

The point is, upon that cut-off at the first branch of the decision tree, my matches leave off well over three hundred results.

Of course, those extra three hundred now-supposed non-relatives clock in at the proposed ranking of fifth cousin to remote cousina classification which already won them Ignore status in my personal decision tree for further contact. This may turn the whole issue into a non-news item for me.

For my husband's results, however, I may see this change a bit differently. He is the one who descends from Perry County, Ohio, ancestorsthose folks in the coal-mining region of southeastern Ohio who can't simply ask a classmate on a date without first checking to see who everyone's great-great-grandparents were. When I find a fifth cousin on his FTDNA results, that match often turns out to be a keeper. We have the paperwork to confirm it. It's nice to be able to get to know these distant relatives-cum-genealogy-aficionados who are as ably equipped to prove the connection as we are. There's a higher confidence level in letting such newfound relatives help fill in the blanks on the descendancy of those connected lines. Those are the matches I'd hate to see disappear, simply because someone decided to champion a different algorithm.

Of course, it's the rare person who wholeheartedly welcomes change, unexamined. I understand the reasons for the change. Besides, it would be nice to exchange that useless padding of umpteen misaligned "matches" for some real onesif that is at all possible. Time will tell. And, from the looks of it, we won't have long to wait to find out.

1 comment:

  1. As someone that daily struggles with units of measure (clinical trials and lab measurements), I see a new unit of measure to me! CentiMorgans.... Hmm...


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