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Family genealogies versus DNA results

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  • Family genealogies versus DNA results

    Another man and I share the same surname, which our respective genealogies both indicate was inherited from a common male ancestor born in 1700.

    We've each had Y-SNP testing done (one by FTDNA and the other by Full Genomes), and match on all but 7 SNPs. I'm assuming these would be the 7 most recent.

    I picked up the impression somewhere that new SNPs occur on average every fourth generation. Is this correct?

    The church and secular records of our branches of the family in North America, and previously in Switzerland back to 1559, indicate that each of its generations were around 27 years in length.

    If everything above apart from the details of the genealogies that we inherited and/or researched ourselves were correct, wouldn't it mean that our most recent common ancestor would have been born not in 1700, but instead sometime during the 1300s - 1400s (i.e., 7 X 4 X 27 years)? If not, is my math off, or am I just thrashing around here?

  • #2
    Mutations happen at any given time, Rates are based on averages.
    One can have lower then average, average, or higher then average mutation rates

    My Father and his paternal 1st Cousin are a GD of 4 @ 67 markers,
    There are 3 markers, one has a two step mutation, that have either mutated when passed on to my Grandfather and or when passed on to my father.
    Cousins yDNA signature is closest to our other surname matches, He is GDs of 0 and 1 to many while my father is GDs of 4 to 5 to same matches.

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    • #3
      Originally posted by delegz View Post

      I picked up the impression somewhere that new SNPs occur on average every fourth generation. Is this correct?
      Somewhere, there was a number 144 years. Still, 4 generations is a good over-the-thumb rule. Different studies, different numbers, probably.
      Maybe there is a trick regarding Math:
      Remember, there are two branches, snp-mutations may have happened in both of them, you only need 4 SNPs in one branch, and 3 in the other branch.
      4x4x27=432
      3x4x27=324
      Close enough to real life, this way.
      (Re-check the numbers, please. Don't trust me.)

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      • #4
        The idea of a biological (mutational) clock is appealing, x number of mutations per generation or per century. However, it is only going to be valid if you look at a very large number of genealogies. It will very likely not be valid for MOST individual genealogies. It is best to resist reading too much into the data (and that includes an exact number of generations), since real mutations can occur at any time, and are sometimes quite complex.

        There are two things to consider here: First, when the samples that are believed to be from two distant cousins from a shared ancestor circa 1700 are found to be a "match" but with some number of differences, you want to be sure the samples really do come from the same branch of the Y haplotree. I assume you will look carefully at the branching structure of the haplotree, as well as the raw data for each sample (from different vendors in this case) to be sure where each sample should be placed in the haplotree. You will have to consider whether any of the non-matching SNP's are due to lack of testing, or from data quality issues.

        Second, how does the genetic data relate to the paper trail? Can you identify any other kits that match one or the other of your kits at least as well? Do you find any exceptions, such that an apparent descendant of the proposed common ancestor failed to match as expected? Can you identify anyone else who should match, that you might be able to convince to take the same test?

        The more tests, and the more detailed your analysis of the raw SNP data, the better you will be able to understand the genealogical implications of the tests.

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