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Tandem mutations on 37 markers?

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  • Tandem mutations on 37 markers?

    I have been mulling this over for some time and I am not happy with this.

    My 4th gr gf Thomas Abrams had several sons. Joseph, Basil, Henry, Gabriel. We have about 15 testers descended from nearly 15 different grandsons of Thomas via the four sons.
    A 16th test is based upon a related line, brother of Thomas.

    At 37 markers we appear to break into two groups, based upon the value at DYS 570 (red)
    Those who are 17 (7 Henry testers, and 1 Gabriel)
    Those who are 16 (5 Joseph testers, and 2 Basil) - colored purple on the YDNA chart.

    17 appears to be the normal value. I believe Joseph and Basil have the mutation.

    This would suggest the exact same marker, DYS 570, mutated for two brothers in the same generation, and mutated the same way.

    In my mind, the odds of the same marker mutating for two out of four brothers, Basil and Joseph, would be extremely unlikely. I would not expect the exact same mutation for two brothers.

    I would expect one brother to mutate. Since mutations are independent, random events, I can not explain how the result from two brothers seemed to have mutated in tandem.

    Is it normal or reasonable to find two brother's mutating in the same way? currently rows 21-34. Not all tests show up consistently.

  • #2
    I think there would be at least a theoretical possibility that a mutation could occur early enough in development (when the tissue destined to become testes consists of only a few cells) to result (much later) in a significant portion of sperm cells eventually carrying the mutation. It might be interesting, though expensive, to compare Big Y results for at least some of these samples.


    • #3

      I do not think you could make such assumptions. STRs do not tend to have normal values (OK, non-null is normal ).

      STRs are subject to random mutations, and that is what makes them suitable to investigating how close the relationship is (and only with certain probability).

      If you are trying to prove or disprove siblingship with Y DNA, neither STRs nor SNPs can give you a positive answer.

      Mr W


      • #4
        I concur with John McCoy, that the most likely scenario is that the mutation happened in Thomas Abrams during his early embryonic development, when a small population of founder cells becomes determined/designated as the future germ cells ("primordial germ cells") that will produce all sperm. Thomas Abrams testes were probably genetically mosaic, with some significant proportion of sperm stem cells carrying the DYS570=16 mutation while the rest were the original value 17.

        A mutation can happen at any cell division and in this case it was probably during the relatively brief set of cell divisions when the germ cell founder population is still very small. Mostly we see them happen at later cell divisions, because there are so many more of those divisions, so the mutation gets limited to only a single offspring. Germline mosaicism is not common in normal circumstances, but it's far from unknown either. ( )


        • #5
          Okay, thank you

          So there is a biological explanation that could explain this.

          Mathematically speaking, it seemed extremely unlikely.

          I am very confident that Thomas is the actual father to the four boys. Although I am lacking a will, etc. But Mr W, how would go about proving siblingship?

          If anyone else has dealt with the mosaic issue, I would appreciate more feedback. Thank you "mouseinhouse" for your explanation.

          John, how would big Y help? None of us have done it, altho I did do a SNP pack (R-M269 to R-Z8).


          • #6
            Offhand, I don't see a clear way to "prove" that the two mutated lines are from ordinary siblings rather than from a single line or from identical twins, because I think there are alternative explanations for every possibility. There's at least a theoretical possibility that the Big Y test could turn up some "private" mutations in each branch of the family, but I can't quite visualize a situation in which Big Y results would resolve the questions you posed.