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FTDNA TiP Report Generations

Discussion in 'DNA Questions and Answers' started by DavidK, Nov 11, 2020.

  1. DavidK

    DavidK LostCousins Member

    The FTDNA TiP report has an option: "Person 1 and Person 2 did not share a common ancestor in the last N generation(s)." If I select 5, for example, the report shows probabilities starting at generation 5. This seems confusing to me, as I've explicitly excluded the last 5 generations. Am I misinterpreting this? Being too pedantic?

    I read an article on a one-name genealogy web site that advised looking at the probability increments between each generation as a way to determine the most likely common ancestor's generation. Has anyone had any luck with that approach?
  2. peter

    peter Administrator Staff Member

    I think they're counting the current generation as the first.
    I wouldn't recommend it. If I throw two dice it's most likely that I'll score 7, but it's still only a 1 in 6 chance - and in that example there is absolute certainty about the odds. DNA mutation is much more complex.

    Furthermore there is the added complication that the common ancestor might be 8 generations back for you but 12 generations back for your cousin. Or vice versa.

    It's not worth spending time figuring out how you are related - just accept that you are, and hope that evidence comes to light in the future.
  3. DavidK

    DavidK LostCousins Member

    That recommendation seems to be strongly supported by the data.

    I have three Y-DNA tests from my mother's family. Two are her male cousins, Patrick & Charles McAtamney, whose line we've traced back four more generations (to their 2G, my 3G grandfather William).

    The third Y-DNA test, just in, is from Seamus McAtamney, whose 2G grandfather, Eugene, was a peer of my 3G grandfather William.

    William and Eugene were both born around 1816 in the same townland in Ireland. Eugene's father is known to have been called William, and William does indeed show up in the freeholders lists for the time period (alongside one other McAtamney man named Felix).

    The Y-111 tests show a genetic distance of 4 between each of Patrick, Charles and Seamus, which was surprising. The Y-DNA of Patrick and Charles had mutated as much between one another as it had between each of them and their 4th cousin Seamus.

    Autosomal DNA tests, not to mention all the historical records, showed that Patrick and Charles are indeed first cousins, but their Y-DNA TiP report, which I posted about some time ago, was not very useful, with probabilities spread across several generations.

    The TiP reports for Patrick/Seamus, and Charles/Seamus, on the other hand, both indicated a high probability that they were related within a few generations of their earliest known ancestors.

    Unfortunately, Irish records from the early 1800s are few and far between, especially for the common folk. It's tempting to draw inferences from what little data there is.
  4. peter

    peter Administrator Staff Member

    Mutations are random. If I told you that I tossed a coin and that is came up heads 4 times, you wouldn't be able to work out how many times I'd tossed the coin in all - it could be anything from 4 to 20 or more, and that's the same problem we have with Y-DNA distances.

    Nevertheless, in an Irish context the good thing about Y-DNA is that it won't be skewed in the way that autosomal DNA is.

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