Every now and then, when I’m doing DNA reports, I run across the perfect example of a DNA phenomenon. Today, it was a mitochondrial DNA mutation in motion. Let’s take a look at what happened, how it was discovered and what it means.
I was contacted a few weeks ago by someone I had been working with on another project. This woman, we’ll call her June, was concerned because both she and her maternal first cousin, Doris, had both taken mitochondrial DNA tests at Family Tree DNA and they didn’t match each other. I took a look, of course, and sure enough, at the HVR1 level, there was one mutation difference, at location T16271C.
This was particularly interesting, because at the first cousin level, these women shared a maternal grandmother, which means that either June’s mother or Doris’s mother had had a mutation in their mitochondrial DNA, or June or Doris did. June asked me how she could tell who had the mutation.
I asked if either June or Doris had siblings. June had a brother, John, so she ordered a kit for John. If John matched June, then their mother is the one who had the mutation. If John matched Doris, then June herself had the mutation.
How do I know this, that the mutation didn’t happen in Doris or her mother? Because the mutation is not “normal” and is listed in the RSRS values in the “extra mutations.”
Furthermore, Doris, who did not carry the extra mutation, had 13,204 matches at the HVR1 level (haplogroup H), where June who did carry the extra mutation only had 41. Clearly to be useful, genealogically, this test would need to be expanded to the full sequence level.
So June’s brother, John, tested and he matched his sister June, telling us that their mother carried this mutation, and gave it to both of her children. So the mutation occurred between June’s mother and June’s grandmother.
Are These Matches Valid?
June asked me if her matches were valid.
That’s a tough question to answer, because convergence has occurred.
So let me answer this in two ways.
The matches are technically accurate. This means that indeed she matches all 41 of the people that the matching routine shows as her exact HVR1 matches. So in that way, those matches are accurate, but they aren’t valid or meaningful for genealogy.
They aren’t useful, because we know, beyond a doubt that these matches are not related to her in a very long time, probably back into prehistory, because the reason she matches them at the HVR1 level is because she just happened to have the same mutation that all 41 of them carry. Carrying the same mutation does NOT absolutely mean you share a common ancestor who carried that mutation. Mutations can occur at any time, and if a mutation happens at this location in the mitochondrial DNA, there is a 1 in 3 chance the person who has the mutation will have the same value as you, since there are only 4 choices, T, A, C, and G, to begin with. This is what we call convergence, and you’ve just seen it happen. People match each other, but because they happened to have the same spontaneous mutation, not because they share a common ancestor who had that mutation. Most of the time, we don’t know whether we are looking at real matches or matches by convergence, but this time, we know for sure, because we can prove that June’s grandmother did not have the mutation, because June’s first cousin, Doris, does not.
So, if June’s HVR1 results aren’t useful to her, whose are? That’s easy, her cousin Doris’s results are representative of the mitochondrial DNA of their mutual grandmother, so Doris’s matches are actually June and John’s ancestral matches as well.
Could There Be A Fly in the Ointment?
Not matching someone you thought you should match is unsettling. Could we test someone else to be absolutely positive we’re not dealing with a back mutation?
Certainly, if grandmother had another female child who had children, or if grandmother has a living male child, they can be tested too. The test on the third child would positively confirm grandmother’s mitochondrial DNA values.
Could we prove positively that the first cousins are actually first cousins, to remove any nagging doubt?
Certainly, using the Family Finder test.
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