Hi, Cynthia.
If you're familiar with matching from autosomal DNA tests, it takes a bit of a shift of thinking when looking at mtDNA. Mitochondrial DNA and yDNA are actually easier to evaluate--all things considered--because dealing with matches is more binary, more yes or no.
I think the kicker is that term "genetic distance." With an autosomal DNA test, we're used to seeing reports that provide us an estimate of how closely we're related, in terms of cousinship and degree of separation.
Genetic distance with mtDNA and yDNA doesn't work that way, and it can be misleading. A GD of 1, for example, does not mean a 2nd cousin rather than a 1st cousin. What it refers to is a difference in one marker or test value...a copying error, a mutation or polymorphism. A GD of 2 means that two marker values differ between the two kits. It has a bearing on trying to estimate the possible timeframe to a MRCA, but at much longer periods, and in a wider range of time; it's in terms of multiple generations to MRCA, not in terms of degrees of cousinship.
For mtDNA--a tiny genome in terms of number of base pairs when compared to any of the 23 human chromosomes, even the small Y-chromosome--there are three "regions" for testing Hyper-variable Regions 1 and 2 (HVR1/2) and the Coding Region...everything else, what you get with a full sequence test. Matches in HVR1 and HVR2 are either yes or no; you either match completely or you don't. Results in these areas are compared to one or both of two statistical base models to help determine your primary haplogroup.
For the Coding Region, Family Tree DNA will display up to--but unlike yDNA, no more than--three differences. To get a little geeky, there are two markers at locations where insertions/deletions are known to happen at high frequencies (positions 309 and 315) which are completely ignored for matching. So if you ever compare detailed results with another family member's kit and count a different GD than FTDNA reports, this is likely why.
Exact matches with a full mitochondrial sequence are not uncommon. Again, it's very small in terms of total number of base pairs, and it changes infrequently. The go-by estimates that Family Tree DNA uses for a full mitochondrial sequence exact match are:
- 50% confidence level that the MRCA is within 5 generations (call it 125 to 150 years)
- 95% confidence level that the MRCA is within 22 generations (call it 550 to 650 years)
So the net message is the FMS exact match you have on that kit is solid proof that the two people share a common ancestor in the maternal line, but it is not proof that they share a recent ancestor until the corresponding, corroborating paper trail of traditional genealogy can be documented. That common ancestor might be as close as one or two generations, or as distant as the 15th century. But it's an important, new piece of evidence!
