Remember the exceptions
The issue has just come up again in reader questions about DNA testing for the direct female line and in Facebook threads… so The Legal Genealogist will once again sound the caution:
Beware the myth of the mtDNA GD0.
That’s a zero at the end, and the GD stands for genetic distance — by definition, “the term used to describe the number of differences or mutations between two sets of … DNA test results.”1
So an mtDNA GD0 means two identical sets of test results: the two people who’ve tested have identical mitochondrial DNA (mtDNA) — the kind of DNA we all have, that we inherit from our mother, that our mother inherited from our grandmother, and our grandmother from our great grandmother all the way back through time in an unbroken female line.2
That identical result is what we hope for when we test the entire mtDNA in what’s called a full mitochondrial sequence. We want that result because, when we get that GD0 match, we can be reasonably sure that — somewhere back in time — we and the person we match will share a common female ancestor.
So… what if we don’t get that GD0 match?
What about the people who match us at a genetic distance of 1 or even 2?
So many times you’ll hear someone say to ignore anything but a GD0 match. Mitochondrial DNA is so stable — it changes so little over the generations — that, we are told, anything that isn’t a perfect match is going to be so far back in time that we’d never get any genealogically valuable data by pursuing it.
Um… not so fast.
That’s the myth of the GD0.
And, like so much else that we encounter in genealogy — and especially in genetic genealogy — it ain’t necessarily so.
Take a look at this chart, of my own family:
The four women identified in yellow have all done full mtDNA testing. All of us descend not just from a common female ancestor, but from a very recent common female ancestor: my grandmother, Opal (Robertson) Cottrell (1898-1995). And since mtDNA is passed from mother to daughter to daughter, all four of us should share identical mtDNA: my grandmother passed her mtDNA to her daughters, my mother and my aunt, my mother’s sister; my mother passed it to my sister and to me; my aunt passed it to my cousin.
We should all be GD0 from each other, right?
But we’re not.
I am a GD1 — a genetic distance of 1 — from my sister. Despite the fact that, yes, we are full blood biological sisters, we do not have perfectly matching mtDNA.
I have something called a heteroplasmy in one part of my mtDNA. That, by definition, is “the presence of a mixture of more than one type of … mitochondrial DNA … within a cell or individual.”3 Where the test expects to see one marker, I show two. That is counted as a genetic difference, and I have one such difference from my sister, who doesn’t have that heteroplasmy.
I’m also a GD1 from my aunt. She doesn’t have that heteroplasmy either. So she and my sister are GD0, and I’m GD1 to each of them.
My cousin, meanwhile, also has a heteroplasmy, but a different one from the one I have. That means she is a GD1 from her own mother and a GD1 from my sister, her first cousin.
And — because each heteroplasmy counts as a genetic difference, and I have one and my cousin has another — my full maternal first cousin and I are a GD2 from each other.
Think about that for a minute.
Would anybody seriously argue that there’s nothing genealogically relevant that two sisters might be able to share with each other — even though those sisters are not GD0 mtDNA matches? Or a mother and daughter who are not GD0? Or first cousins who are GD2 from each other?
So don’t automatically write off those mtDNA matches who aren’t GD0.
Now don’t get me wrong here. I’m not saying to spend a lot of time chasing distant mtDNA matches. They certainly won’t be the ones you focus on first. They may in fact be so far distant that you’ll never find any genealogically relevant information by contacting them.
But then again they may end up being the exceptions… the ones who really are close enough to be not just genealogically relevant but genealogical goldmines… the one who, if you ignore them, could leave you — and them — as victims of the myth of the GD0.