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Testing the cousins

One piece of advice you hear over and over in autosomal DNA testing: test as many relatives as you can afford to test.

And boy did The Legal Genealogist get handed the best evidence in the world as to why that’s good advice this past week.

Now before we go on, the reminder: an autosomal DNA test is the kind of test that works across genders and helps you find cousins in recent generations.1 Unlike YDNA, you don’t have to locate sons of sons of sons to test and only get results in the male line,2 and unlike mitochondrial DNA, you don’t have to locate daughters of daughters of daughters and only get results in the female line.3 With autosomal DNA, you can test the son of a daughter of a son against the daughter of a son of a daughter and get good results even across genders.4

But while you can test more people against each other to compare their autosomal DNA, you also have a shorter time frame for the results to be useful: unlike YDNA or mtDNA, which can be basically unchanged for hundreds, even thousands of years, autosomal DNA changes very rapidly from generation to generation through a process called recombination.5

In a nutshell, each parent 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.6

Now 90% odds at the third cousin level are pretty good. So I was really delighted when, recently, we identified a third cousin in my Cottrell line.

He and I share a great great grandfather — my nemesis George Washington Cottrell.7 I descend through G.W.’s oldest son, Martin Gilbert Cottrell. This cousin, whose first name is David, descends from the youngest son, another George Washington Cottrell. His grandmother Luda Pearl Cottrell and my grandfather Clay Rex Cottrell were first cousins; his mother and mine second cousins; so he and I are third cousins.

But David had tested with AncestryDNA… and so have I… and we don’t show up as matches there. He matches a Cottrell cousin who’s also tested with AncestryDNA but not me.

I couldn’t believe it. I asked him if he’d be willing to test again at Family Tree DNA where the robust analysis tools would tell us so very much more than the “your tree matches his tree” (or not) results at Ancestry. He was willing, the test went in, the results came back this past week and…

He and I don’t match.

Now lest you sit there knowingly smiling to yourselves about some unreported non-paternity event, let me hasten to dissuade you from heading off down that blind alley. David matches both of my Cottrell uncles and my Cottrell aunt. He matches my Cottrell first cousin. He matches both of my Cottrell-side second cousins. He even matches my nephew, for cryin’ out loud.

He just doesn’t match me.

In other words, the key pieces of DNA that allow a positive match between this particular cousin and his relatives in my direct line happened to get passed down to just about everyone in my family… and to miss me.

To see this, take a look at the graphic with this blog post. You can click on it to see it bigger. This view of the chromosome browser shows in orange how my nephew and I match. You can see all those orange-colored chunks on just about every chromosome. These are chunks he would have gotten from his mother, my sister.

Now look at the chunks in blue. That’s where my nephew and my third cousin David share those very particular Cottrell segments that allow the testing company to be able to declare them a match. You can see that there is no orange in those areas.

David got those DNA segments. My nephew got those segments. My aunt, my uncles, my first cousin, my second cousins — they got those segments.

I didn’t.

In a situation where the odds are 90% in my favor… I landed in the 10%.

Maybe the DNA I got in those areas came from my grandmother and not my grandfather. Maybe it came from a different side of my grandfather’s family than the side that shows up here. Whatever the explanation, my DNA in those particular key locations just isn’t the same.

Now think about that for a minute.

If I was the only member of my family who’d been tested, and I was just starting out with autosomal DNA testing, can you see how I could have been misled by these results? It would have been so easy to think David and I weren’t related just because, in the random genetic crapshoot of autosomal DNA, the DNA dice just didn’t happen to roll the same way for the two of us.

I wouldn’t have known that it was just my DNA that differs, and in just the right (or wrong) place to make me the one who didn’t get the segments all my other relatives did.

So the moral of this story is: widen the net. If you’re going to do autosomal DNA testing, test all the relatives you can afford to test.

And don’t be discouraged when — not if, but when — you and a third cousin just don’t share enough DNA in common to be declared a match.

It happens to us all.


  1. See generally Judy G. Russell, “Autosomal DNA testing,” National Genealogical Society Magazine, October-December 2011, 38-43.
  2. See ISOGG Wiki (, “Y chromosome DNA test,” rev. 21 Jan 2013.
  3. See ISOGG Wiki (, “Mitochondrial DNA test,” rev. 29 Jan 2013.
  4. See generally Russell, “Autosomal DNA testing.”
  5. See ISOGG Wiki (, “Recombination,” rev. 12 Jan 2013.
  6. Russell, “Autosomal DNA testing,” at 39.
  7. See e.g. Judy G. Russell, “Oh George… you stinker!,” The Legal Genealogist, posted 9 Jun 2012( : accessed 6 Apr 2013).
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