Genetic genealogy produces much sales patter and much jargon. mixing royal lineages with “SNP”s, “STR”s, “haplogroups” and what have you. But the principle behind Y-chromosome genealogical DNA testing is simple and logical. The Y- chromosome is passed intact from father to son. Life being messy, a tiny random mutation will occur every so often. This is then incorporated into the chromosome of all the male descendants of the man in whom the mutation first arises. So if you test a large group of men for that particular mutated gene, everyone who tests positive must be descended from the original man in whom it cropped up. The test simply identifies the most recent common male ancestor.
So far, so good. There are some very clear uses for such testing – investigating whether a surname is likely to derive from one man or more than one, combining multiple tests to reconstruct probable population histories, even identifying what geneticists delicately refer to as “non-paternal events”. But the accuracy of the tests can vary wildly. If the size of the group being tested is not large enough, or the mutations tested for are not actually unique, the supposed “most recent common ancestor” can be as illusory as Santa Claus.
Much of the hoopla around Irish DNA testing was set off by the well-known TCD study that supposedly identified Niall of the Nine Hostages as the common ancestor of men bearing surnames traditionally regarded as being of Uí Néill origin, such as Gallagher, Bradley, Boyle and Doherty. It turns out that a grand total of fifty-nine men were tested, and that the constellation of mutations used to distinguish them also occurs in other Irish population groups that could not possibly be descended from Niall. (See tinyurl.ie/9bg).
Beware of scientists who expect rigorous scepticism to be applied to scientific research, but think of genealogy as a sub-branch of the jarveys-and-heraldic-teatowel industry.
['Irish Roots archive from 2009 at www.irishtimes.com/ancestor/magazine/column]