Expanding the concept of generation/time to last common ancestor

[jclark343]

The Genetic Distance Tables and accompanying information provided by FTDNA has limited usefulness. Based on 12 markers, I have over 1000 exact matches in the database, which means that our last common male ancestor lived 12 generations ago - or about 300 years ago (70%) to 24 generations ago - or about 600 years ago (90%). Unfortunately most of those 1000 males have a different surname and many with the same surname come from the wrong country. At 25 markers, I have one exact match - with a surname that I don't recognize - and about 600 matches at a genetic distance of 1 or 2. This leads the conclusion that we shared a male relative between 300 years ago (50 to 80%) and 600 years ago (90 to 98%). At 37 markers, I have one match at a genetic distance of 4 with a surname that I don't recognize. This person and I may have shared a common ancestor 300 years ago (58%) or 400 years ago (70%) or 600 years ago (99%). The possibility that the grandfather for me and all of these various matches may have been among the first Celts in Northern Ireland or even one of the first Celts to arrive in Ireland thousands of years ago.
What I would like is some help in estimating far back the common ancestor for me and my matches with a genetic distance of say 10 based on 37 markers might have lived. Was he pre-Roman?
If the genetic distance is 20 to 30 based on 37 markers does that mean our common ancestor was probably a farmer in what would become Germany?

[dna]

That probably should be a FAQ

STR markers are not good at larger genetic distances. And, as you had just learned, even with small distances you need many of them, let's say at least 37 . Regardless of the number of markers, STRs do not allow for a relationship tree to be drawn.

For evaluating the distant common origin/ancestor, SNPs are better, since a tree can be drawn. However, we are not there yet to do that for the past centuries, only for the past millennia.

W.

[jclark343]

Thanks for the insights. I believe, however, that given the state of available information 25 markers might be sufficient. They seem to be sufficient to rule out any recent common ancestors. Investing in more markers simply makes the conclusion more solid.

[dna]

Thanks for the insights. I believe, however, that given the state of available information 25 markers might be sufficient. They seem to be sufficient to rule out any recent common ancestors. Investing in more markers simply makes the conclusion more solid.

At this moment, 37 markers to be a good starting point, better than 25.

I know, early this century only 12 markers were available

W.