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More reasons to test cousins

Once more into the breach, dear friends. One more thoroughly annoying example of why it is so important to test as many cousins as you can afford to test if you’re going to get into trying to use autosomal DNA to help in your genealogical research.

You may remember just a few short weeks ago I wrote about meeting my cousin George Lowe from Texas, a cousin on my maternal grandmother’s side. In my enthusiasm about meeting George, I mistakenly identified him as my second cousin; he’s actually my third cousin. Our relationship can be seen on this chart:


One thing George told me when he was visiting was that he’d taken the AncestryDNA test. The AncestryDNA test, remember, is an autosomal DNA test, and that’s the kind of test that works across genders and helps identify cousins in recent generations.1

That’s the plus side of autosomal DNA. The minus side is that the window of opportunity to get a match can close down pretty quickly between those cousins. That’s because, unlike other types of DNA, autosomal DNA changes very rapidly from generation to generation through a process called recombination.2

In a nutshell, each of your parents has pairs of chromosomes but will only pass one of each to you. Before that happens, each pair the parent has gets mixed together and jumbled randomly so what you end up with may be more or less from any one ancestor. That’s why even brothers and sisters don’t have the same DNA: though each child gets 50% from each parent, each won’t get the same 50% as any other child.

Because of these changes, you get a rapid drop-off in the odds that you and a relative will actually share enough DNA in common to show up as matches. With a close relative, say through second cousin, the odds are pretty close to 100% that you share enough to match. Third cousins will match about 90% of the time. By the fourth cousin level, your odds are only 50-50; nine out of 10 fifth cousins on average will not share enough DNA to match; and by the sixth cousin level it’s really a crap shoot.3

But hey… this was a pretty safe bet, right? Third cousins. Ninety percent. Nine out of ten.

And, late last night, I got an email from another cousin — my first cousin, who like me is George’s third cousin (her mother and mine were sisters) — who said George had popped up as a match in her list at AncestryDNA.

Now you can already see where this is going, right?


George is not on my match list.

Now stop and think here for a minute. If the only cousins who’d tested were George and me, what might we think after finding that we’re a mismatch? Without understanding that autosomal DNA is passed down randomly through the generations, we might very well think one of us had better have a long talk with our ancestors…

But all this means is that whatever genes George and our cousin both got from our Robertson second great grandparents didn’t make it through the recombination process that resulted in me. I’ve already arranged to have George’s results transferred to Family Tree DNA where we’ll be able to see exactly where those genes are (a feature lacking at AncestryDNA) that they have and I don’t.

Still, it sure highlights why it is that — over and over — we’re told that the best advice for autosomal DNA testing is to test everybody you can afford to test.


  1. See generally Judy G. Russell, “Autosomal DNA testing,” National Genealogical Society Magazine, October-December 2011, 38-43.
  2. See ISOGG Wiki (, “Recombination,” rev. 20 Jul 2013.
  3. Russell, “Autosomal DNA testing,” at 39.
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