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  • Genetic Drift, a Max question?

    Genetic drift occurs when two people who are unrelated in a genealogic time frame have their alleles drift into a match. I assume the opposite event can and does occur --where two people who are actually related within a genealogic time frame drift out of a match? How common is this occurrence?


    Also, I have 12/25 matches in my surname, but no more than that, no 24/25 or 25/25 matches. Why not? Are we still related?
    T.I.A.
    John

  • #2
    Hi John. I'm not Max, but let me give this a try. Your statement "when two people who are unrelated in a genealogic time frame have their alleles drift into a match" is a definition of convergence rather than genetic drift. Genetic drift is a more general concept with a definition something like "random changes in allele frequencies from one generation to the next because of small population size." (I was greatly surprised not to find a definition in the usual DNA dictionaries online, but Google found several.) When two haplotypes match due to convergence they are identical by state rather than identical by descent.

    In my opinion, in a genealogical time frame, if two men with the same surname have the same haplotype, it's simply because there have been no mutations since the common ancestor. There could have been a mutation and then a back mutation, but that's probably a small probability. The possibility of convergence from unrelated ancestors can be ignored, especially with 25 or 37 markers.

    Your statement and question "I assume the opposite event can and does occur --where two people who are actually related within a genealogic time frame drift out of a match? How common is this occurrence?" is the normal situation. In our group of Hursts from Virginia dating back some 300 years, about half have one or two mutations from our known ancestral haplotype. Two of the closest known cousins actually have a genetic distance of three, although each is only one or two away from the ancestral haplotype.

    As for "Also, I have 12/25 matches in my surname, but no more than that, no 24/25 or 25/25 matches. Why not? Are we still related?" do you really mean that the matches are 12 markers out of 25? If so, you are not related in any reasonable time frame; surely not since surnames have been used. Thirteen mutations takes too long.

    Hope this helps.

    Bill Hurst

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    • #3
      Hi, Bill. Thank you for the reply, very helpful and informative.

      Originally posted by Bill Hurst
      As for "Also, I have 12/25 matches in my surname, but no more than that, no 24/25 or 25/25 matches. Why not? Are we still related?" do you really mean that the matches are 12 markers out of 25?
      Bill Hurst
      Yes, the matches are only 12 of 25 and none extend beyond 12 markers within my surname. At 25y resolution, the 12y surname matches drop out. I have four 24/25 matches with different surnames at 25y resolution.

      I am correct in understanding that matches of 24/25 with genetic distance of one --all different surnames-- should not be considered as relatives within a genealogic time frame? I am of a common haplogroup, R1b, so this complicates matters.

      The reason I ask is because I am grasping at straws. The paper trail is rather thin for my father and his ancestors and I am wondering if there may have been a surname change somewhere along the way.

      Thanks,
      John

      Comment


      • #4
        Hi again John. Many of those who match you at 12 markers, but drop out at 25, just may not have tested beyond 12. You might want to ask them to upgrade.

        I would contact any 24/25 matches to see if you can find geography in common.

        Surname changes do happen. My mother's family translated their name from German when they got here, then 250 years later added an "e" to the end. Non-paternity events and adoptions also happen. I have a 37/37 match with a person with a different surname. I'm also R1b, but far enough away from the Atlantic Modal Haplotype that a perfect match is a cousin.

        Bill

        Comment


        • #5
          Hi, Bill.

          Originally posted by Bill Hurst
          Hi again John. Many of those who match you at 12 markers, but drop out at 25, just may not have tested beyond 12. You might want to ask them to upgrade.
          Bill
          I forgot to mention that I and they have all tested at 25 or greater markers.

          Originally posted by Bill Hurst
          I would contact any 24/25 matches to see if you can find geography in common.

          Surname changes do happen. My mother's family translated their name from German when they got here, then 250 years later added an "e" to the end. Non-paternity events and adoptions also happen. I have a 37/37 match with a person with a different surname. I'm also R1b, but far enough away from the Atlantic Modal Haplotype that a perfect match is a cousin.
          Bill
          How did you measure your distance from the Atlantic Modal Haplotype so
          that you consider yourself "...far enough away..." from that haplotype?

          Thanks again for all your informative replies,
          John

          Comment


          • #6
            Originally posted by john.boyd
            How did you measure your distance from the Atlantic Modal Haplotype so
            that you consider yourself "...far enough away..." from that haplotype?

            John
            Well, to start with, the original AMF was six markers; I differed on three. At 37 markers, there is the so-called Super Western Atlantic Modal Haplotype. See http://worldfamilies.net/Super%20Wes...0Haplotype.htm

            Bill

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