I find Australian friends on a few family-history blogs getting excited about DNA testing. Meanwhile, the great American science scholar and writer Carl Zimmer has just tweeted:
If I ever get a DNA ancestry test, I want @razibkhan to help me figure out what it all means: http://bit.ly/fPTByV
If you take a look at Razib Khan's lengthy and complex analysis of his personal DNA results from 23andMe, you'll realize immediately that Carl Zimmer was being ironic in a friendly fashion. Often, naive newcomers to genealogical testing are awestruck by what the testing firms offer them. Certain testing firms lure their customers on by letting them believe that they're likely to come upon all kinds of cousins in the published databases. It goes without saying (as a little serious in-depth study of the subject, not to mention some basic arithmetic, would rapidly reveal) that these claims are surely exaggerated, to say the least.
All the inherited characteristics that make an individual what he or she is, today, come from a set of ancestors who were present on the planet Earth at various times over, say, the last couple of thousand years. That's already a lengthy time frame, and few of us have serious chances of finding out anything whatsoever concerning individual ancestors who lived, say, at the time of the Roman Empire. Not even kings and queens can obtain that kind of data! Moreover, you've been influenced genetically, during these two millennia, by a staggeringly vast horde of direct individual ancestors. (Do the arithmetic: 2 to the power of G, where G is the generation that interests you. Admittedly, there are countless repeated individuals in this crowd.) Consequently, the genetic input of any particular individual in this horde is like a drop of water in a wide and deep river.
In the domain of Y-chromosome or mtDNA haplogroups, the frame of reference extends back in an awesome exponential fashion over tens of thousands of years, giving rise to an ocean of population demography in which the very notion of your particular ancestors ceases to have any meaning whatsoever. And the particular individuals who provided you with the molecules that you might send off to get analyzed today were like a tiny line of bubbles rising to the surface of this vast ocean.
At a down-to-earth level, I've often said that DNA testing can possibly provide genuine assistance in the domain of genealogical research. In my personal case, for example, if ever I came across published Y-chromosome markers whose values matched mine, and if the individual in question happened to have an appreciable amount of traditional genealogical data about his background, then I might be able to learn more about my male ancestors named Skivington, Skevington or Skeffington. But those are two big "ifs". My results include values for 67 markers. Here's what I'm offered, today, when I look for matches:
Restricting my matching search to a maximum of little more than a third of my 67 tested markers, I find four individuals whose values are vaguely close to mine, with a difference (a so-called "genetic distance") of 3. Insofar as the values of a typical marker mutate extremely slowly (let's say, once every few centuries or so), it's most unlikely that any of those tested individuals named Walsh, Gifford, Davis and McGrath shared an even remote paternal ancestor with me since the end of prehistoric times. Consequently, it would be a pointless waste of time for me to attempt to contact such individuals in the hope of our sharing common family-history information.
So, you might say that my investment in Y-chromosome testing with FamilyTreeDNA was a little like buying a lottery ticket. And I haven't got anywhere near winning a prize yet.
ADDENDUM: Often, I imagine scenarios involving a near-perfect match between my 67 markers and those of another male, somewhere on the planet. The ideal scenario would involve an auburn-haired Frenchman named, say, Jacques Beaumont, living today in Normandy, who would go on to tell me, once we got into contact, that his family had a distant ancestor who went to England at the time of William the Conqueror. I would then be in a position to assume that the ancestor in question was no doubt the fellow who settled down in the Saxon village of Sceaftinga-tûn in the county that became known as Leicestershire. But there are countless other less perfect scenarios (where my use of the adjective "perfect" is deliberately tongue-in-cheekish). For example, once we move back to the 17th century, I no longer have any reasons to believe naively that all my direct male ancestors were indeed bona fide Skivington husbands. When I was an adolescent, the Aussie slang expression "ring-in" designated a substitute, somebody brought into a family context, often on false pretences. (I don't know the origins of this expression.) If, in a family, one of the offspring behaved quite differently to the other siblings, the child might be labeled a ring-in, indicating that the true identity of his/her father was not entirely guaranteed. So, it's quite possible that one of my ancestors was a non-Skivington ring-in who had succeeded in jumping into bed with the current Mrs Skivington and procreating the ancestral line that finally produced me. And we might imagine that this ring-in had a brother who was a seaman working on an old sailing-ship that once ventured out, say, to Batavia (modern Jakarta). While the vessel was picking up spices, the seaman might have picked up a young local lady and got her pregnant. If that were the case, then we could well expect that an Indonesian gentleman, today, has exactly the same Y-chromosome markers as I do. Moreover, there are 16th-century males in Turvey (Bedfordshire) referred to as Robert husbandman Skevington and George husband Skevington. Funnily enough, the term "husbandman" doesn't necessarily designate the chap who was legally married to Mrs Skevington. Etymologically, a husband was a fellow who tilled the soil. So the above-mentioned Robert and George might have been plowmen who worked as agricultural laborers on the Skevington estates in Turvey. In that case, genealogically, they would be ring-ins. So, anything's possible… even with perfectly matching Y-chromosome marker values. To borrow the title of a funny French movie, it would have been nice if the existence of our ancestors had always been like a long and tranquil river.