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An atDNA puzzlement

FTDNA Family Finder on order

It’s DNA Sunday here at The Legal Genealogist and I find myself convinced, as usual, that autosomal DNA testing — atDNA for short, the kind of testing called Family Finder by Family Tree DNA and Relative Finder by 23andMe — as the King in The King and I says, “Is a puzzlement!”1

We’ve tested a fair number of people, both in our immediate family and more distant cousins out to the second cousin level. Among the closest ones tested are my Uncle David (my mother’s younger brother), my first cousin Paula (daughter of David’s and my mother’s youngest sister) and, of course, yours truly — the self-professed DNA geek who’s never met a DNA test I wouldn’t take (and hasn’t yet taken all the ones I’d like to take nor tested all the family members I’d like to test!).

The results can be A Puzzlement indeed. I was looking at them yesterday, and particularly at our matches in common, and realized that Paula and I share a total of 20 matches with our Uncle David. Of those, six are members of our Family Finder group whose tests we arranged. They represent a ton of our various lines:

     • two share our Robertson-Gentry lines;
     • one shares a bunch of lines — Fore, Cottrell, Buchanan, Baker, and Johnson are the closest;
     • one should be only a match to our Fore line — one where we don’t know the wife’s maiden name;
     • one is only in our Battles-Shew-Brewer lines; and
     • one shares the Battles-Shew-Brewer lines and the Robertson-Gentry lines.

But we have 90 people — four and a half times as many as we share — that one of us matches with David — and the other one doesn’t. I have 54 matches in common with our uncle that my first cousin doesn’t share; Paula has 36 that I don’t share.

Now you might be thinking that perhaps the matches in common are really not on our mothers’ side but rather matches that might be influenced by our respective paternal lines. Say, for example, somebody who was far enough away in generations to match David but not me on my mother’s side, and then matched me through a common ancestor on my father’s side. Nice theory, except that my mother’s family has been in the United States since, oh, roughly back when God was in short pants, and my father’s family arrived in the United States in 1925.

If we were going to see that sort of accidental common matching, it’d be in Paula’s matches that we might actually expect it — someone who’s far enough away in generations to match David but not Paula and who then shares enough DNA with Paula’s father to show up as a match to her as well. Her Dad’s family has been in America probably as long as our joint mothers’ family has.

But we don’t see it there at all. I have nearly twice as many matches in common with David as Paula does. David and I share matches with many many more of our Gentry relatives; Paula matches more in what we think will be the Fore-Johnson line. And we wonder why; we wonder if there even could be a reason why, given the randomness of recombination.2 Why did more of my genes come from the same batch that our uncle got, and fewer of my cousin’s genes?

The only thing that seems different between us is our places in our family. My mother was in the first half of her generational birth order, my grandparents’ fifth-born and fourth-surviving child; Paula’s mother was their 12th-born and last of 10 surviving children. (David is the sixth-born and fifth-surviving child.) And I’m way up in the top end of my generation (third of all the cousins) and Paula is next to the youngest. Could it be that birth order played some role in who got what genes?

Probably not,3 but hey… it’s as good a theory as any, and trying it on for size gives me the excuse to round up more family members to test.

Yep, I took advantage of the Family Tree DNA sale yesterday and ordered two more test kits: one for the oldest surviving Cottrell child (other than David), our Aunt Carol; and one for the youngest surviving child (other than Paula’s mother), our Uncle Mike.

If nothing else, it’ll give us more information on our grandparents’ genomes. And that’s not a bad thing at all.


  1. Oscar Hammerstein II, A Puzzlement, The King and I; lyrics at SongLyrics ( : accessed 201 Apr 2012).
  2. The autosomal DNA animation at Sorenson Molecular Genealogy Foundation may provide the single best explanation of recombination. See “Molecular Genealogy: Animations: Autosomal DNA,” SMGF ( : accessed 21 Apr 2012).
  3. See Shuai Chen, “Are your traits equally passed on to each child or would your first child contain more direct features of you then, say your third?,” Ask a Geneticist, The Tech Museum, Stanford School of Medicine ( : accessed 21 Apr 2012).
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