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The downside of DNA testing

Reader Beth Griffin has run into a snag in using DNA testing to help with her family history. She writes:

I am completely new to DNA use in genealogy. I had my dad submit a DNA sample to paternal lineage testing, 46 marker test. It came back and not a single person on the list shares his last name. People who reportedly share a common ancestor with him as recent as 6 generations back (about 150 years) number about 10, with three of them having the same last name but none of these are dad’s. How is this possible and, can you give a tutorial in layman’s terms about the process and what to look for?

Beth, there are a lot of factors at play here but oh yeah… it sure is possible — and not at all unusual — that there is a mismatch between a person’s DNA and that of others with the same surname, even a common surname. The fact that one test can’t always give us the answer we want — and sometimes not an answer at all — is the downside of DNA testing. That, and the fact that, occasionally, DNA testing can tell us something we might not want to know…

First, let’s do a quick trip through the type of DNA test your Dad did. It’s a test of his YDNA, the gender-linked type of DNA that only men have. So it’s a test that traces only the men in your Dad’s line: his father, his grandfather, his great grandfather and so on. All of the male descendants of these men will also have very similar DNA, so your uncles and male cousins and brothers will similarly have the same or very similar results.1

YDNA generally doesn’t change very much over very long time periods, though there may be small differences from generation to generation and even between brothers or cousins. Because it’s usually pretty stable over the generations, it’s a good way to see if two men could descend from a common ancestor.

For a good overview of YDNA testing generally, let me recommend a couple of websites. First, read up on this kind of testing at the Sorenson Molecular Genealogy Foundation. The animation it offers makes it pretty clear how YDNA works. Then check out the YDNA page of the International Society of Genetic Genealogists Wiki. And you really can’t go wrong with Debbie Kennett’s new book on DNA testing2 or with the older but very readable DNA book by Megan Smolenyak and Ann Turner.3

The key thing to remember here are that YDNA by itself doesn’t tell you who your Dad descends from. It’s only when his DNA is compared to the DNA of other men who’ve tested and that you’e compared the paper trails of these men that you really have any useful information.

When I say “the DNA is compared,” the value of the information you get really depends on four things: how many people have been tested by your testing company (how many results are in the overall database), how many specific DNA areas (called markers) are tested for each person, how many markers are the same between any two or more people tested, and whether those tested share a surname.

The size of the database is a big factor: you want to be able to look at as many different results as you can. The fact that your Dad doesn’t match others with the same surname at could mean nothing more than that nobody else who shares your father’s bloodline and his surname has tested with Ancestry.

That being said, Ancestry’s YDNA database is big enough to offer value here. Still, it’s not the biggest around and I’d certainly recommend that you test with other companies if you can — and Family Tree DNA has the biggest database around.4

How many markers are tested is also a big factor, and the more tested, the better. That’s because there’s a huge difference between having, say, 12 out of 12 markers that are the same, and having 67 of 67 the same. The significance of a difference in a marker helps figure out what’s called genetic distance, or the number of differences between two sets of results5 — and there again it depends on the number of markers tested: you’re a lot less likely to be related if you only match in 11 of 12 markers than if you only match in 66 of 67 markers. Ancestry’s 46-marker test is a good one, but again if you can afford to transfer his results over and test with FTDNA, 67 or 111 markers is better.

How big the genetic distance is between two people is what lets the testing companies predict how recently you may find a common ancestor. But I want to emphasize here that any statement as to how recently two people who’ve been tested might find a common ancestor is an estimate, not a guarantee. Because YDNA doesn’t change very much, it’s entirely possible to have absolutely identical YDNA results and yet the common ancestor won’t be recent at all but many many generations back. By the same token, it’s entirely possible for two brothers to have one marker out of 46 that’s different from each other.

The different testing companies report matches a little differently, and Ancestry is generally regarded as being… well… let’s just say a little optimistic in saying how recently two people who’ve tested might find a common ancestor. Even though Ancestry predicts that you should find a common ancestor with these other folks in the last six generations (150 years or so), most other testing companies would report the odds of that being the case as much lower.

For example, several of your father’s reported within-six-generations matches have two of the 46 tested markers that are different from your father’s, or a genetic distance of two. Family Tree DNA estimates that the chances that two people with a genetic distance of two will have a common ancestor within the last eight generations are only about 30 percent. It doesn’t go to 50-50 until about 11 generations, and doesn’t hit 75% until 17 generations, give or take a few years.6

So figuring — say — about 25 years per generation, the odds are only 50-50 that the common ancestor between your father and these folks was alive 275 years ago and there’s a one-in-four chance that the common ancestor lived more than 400 years ago.

As you note, however, there are three different people who’ve all tested with Ancestry who all share a last name that’s different from your Dad’s, and they’re all at a genetic distance of one from him. According to the Sorenson folks, that “may indicate a surname change in one line or may be coincidence.”7

To really understand what a surname change means, we need to factor in the history of the use of surnames. Though we can’t imagine anyone not having a surname today, it’s actually a fairly new phenomenon, dating back to only around 1500-1600 for us common folk.8 The poorer and more rural our ancestors, the later historically they were likely to have adopted and used a surname.9

And, when surnames were adopted, there was nothing to stop two brothers from adopting different surnames. Just as an example, one brother might have become a blacksmith and so might have chosen to use the surname Smith. His brother might have taken up farming the family plot and might have chosen simply to be known as the son of his father Will… in today’s terms, Wilson.

Now I don’t want to suggest that surname choice a long time ago is the only reason why two men today might test the same or very similarly and yet not share a surname. Whenever we see this, we also have to consider what’s called a non-paternity event (NPE).

This does NOT mean that one of the mothers somewhere in one of the lines was foolin’ around on the side. Sure, it’s possible that an illegitimate child would use his mother’s surname — I have that situation in my own family.10

But there are many more common explanations for a surname change than illegitimacy. It was very common for a child who was orphaned to be raised by relatives, and that child often chose to use the surname of the family who raised him. A nephew raised by his mother’s brother or his father’s sister’s family, a grandson raised by his mother’s father, might well have used a surname other than the one he was born with. It was also very common for widows with young children to remarry, and for their young sons to use the surnames of their stepfathers.

Was there an NPE in your Dad’s genetic history? I ran that question past two of the smartest genetic genealogists I know, CeCe Moore, who blogs as Your Genetic Genealogist, and Blaine Bettinger, who blogs as The Genetic Genealogist. Both agree that the answer can only come from more testing… but that there is a chance that there might have been.

Your father’s surname is one of the 10 most common in the United States11 and hundreds of people with that surname have tested with Family Tree DNA and joined the testing project for that surname. Your father doesn’t match any of them exactly. Blaine explains why that’s significant: “If there are 200 people with a surname who’ve been tested and another person with that surname tests and doesn’t match any of the others, this is more suggestive than him not matching 20 people with his surname.” He adds that a better question than who your Dad matches is whether there’s anyone who’s already tested that he should be expected to match and doesn’t — a distant cousin, for example.

CeCe notes that the genetic distance between your father and his closest matches in his own surname project is four12 — which drops the odds of a common ancestor but doesn’t eliminate it. It’s possible that the markers that are different are ones that change much more commonly and more quickly than others — which, she notes, appears to be more the case in your Dad’s surname than in the other surname where he shares a close result. In men with that other surname, there is one marker that all of them seem to share, and your father doesn’t. That, she suggests, could mean it’s really not as close a relationship as the numbers may make it seem.

Bottom line: it’s time for more testing to know for sure. But, just in case, you just might want to start talking to those folks from that other surname to see where their roots and yours might be intertwined.


  1. See generally ISOGG Wiki (, “Y chromosome DNA test,” rev. 23 Jul 2011.
  2. Debbie Kennett, DNA and Social Networking: A Guide to Genealogy in the Twenty-First Century (Charleston, S.C. : The History Press, 2012).
  3. Megan Smolenyak and Ann Turner, Trace Your Roots with DNA: Use Your DNA to Complete Your Family Tree (New York : Rodale Books, 2004).
  4. Family Tree DNA reports that its database contains results from more than 228,000 YDNA tests and that its overall database is larger than the combined databases of all other testing companies. See “About The Family Tree DNA Database,” Family Tree DNA ( : accessed 7 Apr 2012).
  5. Glossary: Genetic Distance, Family Tree DNA ( : accessed 7 Apr 2012).
  6. These figures are drawn from some of my own family YDNA tests at Family Tree DNA and are calculated against people sharing a surname.
  7. Matching Y-Chromosome DNA Results,” Sorenson Molecular Genealogy Foundation ( : accessed 7 Apr 2012).
  8. Wikipedia (, “family name,” rev. 29 Mar 2012.
  9. See e.g. Sean Murphy MA, “What’s in a Surname?,” Directory of Irish Genealogy ( : accessed 7 Apr 2012).
  10. See Friedrike, how COULD you?“, posted 7 Jan 2012, The Legal Genealogist ( : accessed 7 Apr 2012).
  11. Wikipedia (, “List_of_most_common_surnames_in_North_America,” rev. 4 Apr 2012.
  12. Ancestry and FTDNA test different markers so only 32 of Ancestry’s 46 can be directly compared to those who’ve tested at FTDNA.
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