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  • Genetic Distance

    I will appreciate a little more insight into "genetic distance". My understanding is that it's a measure of the number of mutations and that mutations correlate with the number of generations that have passed since the MRCA, i.e., the lower the genetic distance, the closer or more recent the common ancestor. I also understand there is a probability function applied to estimate the strength of the relationship. With that said (and assuming my understanding isn't totally off-base), if I have a genetic distance of "1" with someone, what is the probability that we share a common father? Grandfather? Great grandfather, etc.?

    Thanks for the education.

  • #2
    Depends on the test in question. In the case of Y-STR testing, Genetic Distance is a raw count of the number of Genetic Differences found between two tests. Where if you have a polymorphic marker and have 4 repeats to their 5, you are GD1 from that. If they have another marker the reads 13 to your 12, you gain another "point of genetic distance" between yourself and that match. So you would be GD2 in total.

    STR Matching has the TiP report you can use to "rough in" the mutation rate of the involved markers and calculate probabilities as to when your MRCA is in terms of generations. But with Y-STR testing, you need to be careful as those can BOTH come back closer than they initially appear(I am GD2 with my father by Y37), to much further away than they appear(others have GD0 matches at 67+ markers who are more than 6 generations back for their MRCA).

    STR testing and the GD marker it provides is something to be handled carefully and with consideration towards what the larger picture might look like. Generally speaking, if a person fails to fall withing the GD filters at Y37 or above, they're not likely to be a close relation on the Male line, although they could still be a more distant one. My current terminal SNP matches on BigY for example fall outside the range for STR matching at all levels.

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    • #3
      Thanks for the feedback. In my case, I have a GD of 1 with a couple of individuals via the Y67 test. I'm guessing they're pretty close relatives, i.e., within two or three generations. If I'm correct, obviously a grandparent or great grandparent was doing pushups in the pickle patch with someone that wasn't an authorized participant. That wouldn't surprise me in the least, but I'm questioning the legitimacy of my assumptions.

      Cheers.

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      • #4
        Originally posted by hAltonJones View Post
        Thanks for the feedback. In my case, I have a GD of 1 with a couple of individuals via the Y67 test. I'm guessing they're pretty close relatives, i.e., within two or three generations. If I'm correct, obviously a grandparent or great grandparent was doing pushups in the pickle patch with someone that wasn't an authorized participant. That wouldn't surprise me in the least, but I'm questioning the legitimacy of my assumptions.

        Cheers.
        What does the TiP report say for when they cross the 50% threshold? If it is that recent, autosomal should sort it out with a comparatively small amount of work. But if you're on a more stable Y-DNA line, you might be looking at a couple/few centuries ago rather rather than a handful of generations.

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        • #5
          TiP report says there's a high probability. Unfortunately, I don't believe any of these people have done autosomal. I guess that's my next project - selling them on autosomal. Thanks for taking the time to comment.

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          • #6
            Autosomal is the quickest(FTDNA often has results back in less than 3 weeks), and cheapest good option, otherwise it basically needs a BigY and you're looking at months for results to come back.

            Even a single SNP test could take a few months to return results, although at $40/each they'd be the cheapest option of all, albeit a slower one that has dubious effectiveness. But first you'd need to know which SNP to test for, and that effectiveness would be less than ideal, presuming a positive test result. As many/most Single SNP tests on FTDNA are looking at Y-DNA mutations that are presumed to be centuries or even millennia old. On the plus side, the single SNP test would tell you if STR Convergence was in play. (Big Y looks for new markers which may become a (single) testable SNP in the future, so SNP testing limitations are a bit different on that count.)

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