Help me understand DNA testing

[gilmore]

...
It's also pretty clear that the Y chromosome does not mutate evenly across all loci.

http://www.sciencedirect.com/science...cfeb20edae7dc1


In summary, again, I'd be chary of putting too much weight on the TIMING of things, as "determined" by yDNA testing.

However, one thing you said is spot-on, for sure... "with precision that increases as more and more men test"

I'd also like to very slightly edit another phrase that you wrote, to make a point...

In other words, it is now sometimes possible to say, e.g., that "It is more probable than not that Mr X, Mr Y and Mr Z descend from a common male ancestor who lived on the island of Harris 250-300 years ago."

sometimes...yes. Sometimes, no.

I quite like the way you phrase this...

In other words, it is now sometimes possible to say, e.g., that "It is more probable than not that Mr X, Mr Y and Mr Z descend from a common male ancestor who lived on the island of Harris 250-300 years ago."

It's clear that you're talking about estimates, and understand that generation times can vary. As long as the discussion is about "estimates" I'm good to go!

And you make a good point. As the cumulative data set widens, the accuracy is only going to get better....though it will still always be an estimate.

Yes, of course not all the alleles mutate evenly, and this is taken into account in predicting the closeness of relationships.

And of course we are talking about probabilities. Even the paper trail is only evidence of probability, no matter how well documented. The names, dates, and other info on tombstones is not always accurate. The records in family bibles is sometimes based on hearsay. And the sloppiness of census takers is notorious. See the preceding post about misattributed paternity. Even a wife's statement that her husband is the father of her child is accurate only some 96-96.3% of the time historically in our culture.

It might be best to think about all genealogical research as something akin to the concept in law of the best evidence rule, and our conclusions are based on different degrees of certainty, e.g., "more likely than not" suppositions, as used in civil cases, versus "to a moral certainty/beyond a reasonable doubt," the standard of proof needed in criminal law to convict.

There is no way to know about the our ancestry to the degree of certainty that some seem to want unless we have video footage of all of our ancestor couples having sex, dated to nine months before the birth of their alleged child. As well as continuous footage of the few days before and after, to prove that they were the only two people involved.

Genealogical research is like detective work. We move from clue to clue, fleshing out each, until all other, contradictory conclusions that we can base on each set of facts are shown to be improbable. At least, if we do it well.

E.g., Y DNA testing of men of Asian descent has shown a haplotype that shows a number descend from one common male ancestor who probably lived a bit north of China in the 11th and 12th Centuries. The place is estimated by the geographic distribution of the testees, the time by the mutations, to such a degree of certainty that all conclusions are improbable except one: these men descend from Genghis Khan through his male descendants. We rely not only on pure genetics, but also on our knowledge of history, of the Mongol policy of killing men in the armies of their opponents and taking the women as sexual partners.

There are several of these historical figures whose YDNA can be identified among their living male descendants. Some are perhaps better termed mythological than historical, since we know more of their stories than the facts of their lives. Somerled is one. Another is Niall of the Seven Hostages. One of the more interesting, and probably the oldest, is the Cohen Modal Hapotype, carried by some, but not all, of the men who are or claim descend from that priestly tribe of Jews, the kahane or cohens. The story of how it was identified, the critiques of it, etc is an interesting read. But it is now fairly well accepted. Granted, whether their alleged progenitor, Aaron, brother of Moses, who lived some 3,000 years ago, was an historical figure or whether the bliblical information on him is accurate, is another matter. Nonetheless, the YDNA does not contradict that conclusion.