
Originally Posted by
Maciamo
MtDNA is the easiest way to verify if two individuals are related on their mother's side. But as the mtDNA sequence is much shorter than the Y-DNA the number of generations separating two persons cannot be assessed like with Y-DNA STR's.
If two people have exactly the same sequence of 16,500 base pairs, they are certainly related and belong to the exact same haplogroup and subclade. But haplogroups include people with many different mutations (SNPs). There are only mtDNA 26 haplogroups for all the people in the world, and a few hundred subclades. It's easy to understand that millions of people will share the same haplogroup.
Belonging to the same haplogroup means that you share an ancestor on your matrilineal side (mother's mother's mother...) who lived some time between 10,000 and 70,000 years ago (depending on the haplogroup). Some haplogroups have very deep levels of subclades. For example D4b1a2a1a or J1c3a2a. The deeper the clade and the more recent is the last common ancestor between people in that clade.