Irish Roots

John Grenham


Last week I wrote a little about Y-chromosome DNA testing, generally used to trace paternal descent. The other major test on offer is of mitochondrial DNA, mtDNA. Only men have a Y-chromosome, while everyone possesses mtDNA, but it is inherited only from the mother. So my mtDNA comes from my mother, and my son’s mtDNA does not come from me, but from his mother.

It would appear, then, that mtDNA testing can be used to research the maternal line, in the same way as Y-DNA tests can be used for the paternal line. However, there are significant differences. First, anyone can take an mtDNA test, but only males a Y-DNA test – a woman researching her paternal line will have to get a DNA sample from a close male relative. Second, and more significant, mtDNA mutates much more slowly than Y-DNA, meaning that the resolution of the mtDNA test is a lot less precise: whereas a paternal test can identify the most recent common ancestor down to a few centuries ago, the maternal line is really only accurate at the level of millennia.

So in reality mtDNA testing is beyond the range of genealogy. Its main scientific use has been to trace evolutionary change – the descent of the domestic dog from the wolf has been mapped out, for instance. In human history, its chief contribution has been to clarify the paths that migrations took out of Africa and around the globe over tens of thousands of years.

Because the rate of mutation of mtDNA is slow and stable, this has permitted the confirmation of the so-called “Mitochondrial Eve”, a single African woman living about 100,000 years ago, who is the earliest common ancestor of everyone now living.

What has been called “deep ancestry” is very seductive. It also has dangers, of which more next week.


Comments and suggestions are welcome, to irishrootsatirishtimes.com.

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