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Science and WDYTYA’s “10th” cousin

Okay, this time it isn’t Ancestry’s commercials that annoyed me in Friday night’s episode of Who Do You Think You Are? but rather the science used by Ancestry in the show itself.

I have a bit of an issue with telling person A (Blair Underwood) that person B (Eric Sonjowoh) is a 10th cousin when there isn’t a prayer of a paper trail to support that statement — and the science isn’t good enough to say it either.

Now let me be fair here. The words “10th cousin” never came out of the mouth of the Ancestry representative who reviewed Blair Underwood’s DNA results with him. The words that were spoken were “your cousin” and “clearly you shared a common relative on the paternal side right around 1600 or 1700.”

10th cousin?????

Fine. But what was showing on the screen? I grabbed a screen capture (left) just to be sure I wasn’t seeing things. Yep, there it is, bold as brass: Eric Sonjowoh. 10th cousin.

Now some of the DNA results shown were from Ancestry’s new autosomal DNA testing1 — an as-yet-unlaunched product2 designed to compete with Family Tree DNA’s Family Finder test and 23andMe’s Relative Finder.

But Ancestry isn’t testing any more autosomal DNA than the other companies — the Ancestry scientist mentioned about 700,000 sampled areas, the same as the others — and autosomal DNA testing simply can’t get you back to 1600 or 1700 with anywhere near the kind of accuracy to be able to say someone was a 10th cousin (and not an 8th or 9th or an 11th or 12th or even a 5th or 6th cousin).

The hitch is that autosomal DNA goes through a process called recombination3 — a random shuffling that takes place as that kind of DNA gets passed down through the generations. Once you get past third or fourth cousin, autosomal DNA may be able to say you’re related, but it’s a total crap shoot as to what degree of cousin you might be.

So let’s say the test used to definitively match the two men as linked on their father’s sides was YDNA — a test of the DNA passed from father to son to son. Even that test can’t give you more than a best guess as to how far back in time you and a match might find what’s called your Most Recent Common Ancestor (MRCA). It’s the result of a statistical analysis and mathematical calculation, and only tells you the likelihood that a common ancestor is within a particular time frame.4

Ancestry itself admits as much. It says on its website, discussing the Most Recent Common Ancestor:

MRCA is a term that is used often in discussions of genetic genealogy. MRCA stands for Most Recent Common Ancestor and it refers to a statistical calculation that determines the likely generation in which two individuals share a common ancestor. The MRCA is often reported with a 95% Confidence Interval. The MRCA number itself is the generation in which it is 50% likely that two individuals are related. The 95% Confidence Interval then gives you a range of generation values that encompass 95% of all possibilities.5

Translation: it’s a guess. A scientific guess, for sure. One based on careful study and analysis, for sure. But it’s still a guess.

Part of the problem is that we don’t have nearly as much information as any genetic genealogist would like about the rate at which particular places in the YDNA (a locus, singular, or loci, plural6) may change (mutate) from generation to generation within a specific population. Again, Ancestry admits as much:

The MRCA calculation relies heavily on the mutation rate of the loci. Each marker tested on the Y-chromosome has its own mutation rate, (and) these mutation rates are known for many of the Y-chromosome loci across several human populations… The mutation rate can be affected by several different factors such as population, haplogroup within a population, length of the allele, region on the Y-chromosome, and size of the repeat structure.7

Translation: when you say the mutation rates are known for many of these loci in several populations, you’re also saying they’re not known in others. At best, only some of what we need to know to make good predictions is known, and some isn’t. And there’s a lot of variation.

And then comes the kicker from Ancestry itself: “For those loci where the mutation rate has not yet been determined, DNA has chosen to use a rate of .0028.”8

See? It’s a guess. It’s akin to the old GIGO issue — garbage in, garbage out. In this case, not enough data in, not enough confidence in the data out. If for any single line of YDNA we encounter no mutations at all, two people could look genetically very much alike and yet the MRCA could be very very far back indeed. And if for any other line we encounter more mutations than the average, two people could look less genetically alike than they really are and the calculation of how far back the MRCA is would be skewed in the other direction.

Bottom line: doesn’t matter whether it was an autosomal DNA test or a YDNA test or both. The results can’t show that Blair and Eric were 10th cousins.

Now don’t get me wrong. I loved Friday night’s episode. Great use of original records, wonderful effort not to take the records at face value but to evaluate them in the context of the time and place, and a good dose of cold hard reality in dealing with slavery and slave ancestors. Loved the show.

But c’mon Ancestry… Cousins, sure. 10th cousins? C’mon. Words matter. Even if they’re just on graphics.


  1. ISOGG Wiki (, “Autosomal DNA,” rev. 8 Feb 2012.
  2. See CeCe Moore, “Update on the New Autosomal DNA Test from”, Your Genetic Genealogist, posted 26 Jan 2012 ( : accessed 25 Feb 2012). See also Blaine Bettinger, “WDYTYA Reveals More Information About’s New Autosomal DNA Testing,” The Genetic Genealogist, posted 24 Feb 2012 ( : accessed 25 Feb 2012).
  3. ISOGG Wiki (, “Recombination,” rev. 30 Jul 2010.
  4. If you’re a math geek, and you want to see just what goes into this analysis, see John F. Chandler, “Estimating Per-Locus Mutation Rates,” Journal of Genetic Genealogy 2:27-33 (2006) ( : accessed 25 Feb 2012).
  5. “DNA: Paternal Lineage Test,” ( : accessed 25 Feb 2012).
  6. ISOGG Wiki (, “Locus,” rev. 10 Jul 2010.
  7. “DNA: Paternal Lineage Test,” ( : accessed 25 Feb 2012).
  8. Ibid.
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