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  • Half or full siblings?

    My grandmother was brought up as a child of this family but I was sent a copy of her birth certificate which showed she was born two years prior to the marriage.
    I have DNA matches with two descendants of two of her brothers.
    I am unable to be sure whether they are full brothers or half brothers.
    A- has a total of 79 cM with me - on chromosomes 9 and 15
    B- has a total of 148 cM with me - on chromosome 9, matching me and A and also matching me on chromosomes 2, 8,11.13 and 21.

    A and B share a total of 67.4 cM with each other on 9,10 and 21.

    Bearing in mind that A and B are clear descendants of two full brothers of my grandmother, who is reputed by virtue of the birth certificate to be their half sister I'm puzzled that I have a higher cM total with B than he has with A. Considering that the marriage of their MRCA was in 1898 this total seems low to me.

    Would the fact that B is of the same generation as me and A a generation below be an explanation?

    Does this confirm definitively that my grandmother was the child of a different relationship or does it indicate that she might have been a child of the legitimate marriage of the ancestors of A and B but born prior to their marriage.

    I would be grateful for advice as this is way beyond my level of expertise.

  • #2

    Would the fact that B is of the same generation as me and A a generation below be an explanation?
    Yes, as a rule of thumb, you can expect cca half as much common dna when you look one step down the family tree.
    And more variability, bad news for genealogists.

    Does this confirm definitively that my grandmother was the child of a different relationship or does it indicate that she might have been a child of the legitimate marriage of the ancestors of A and B but born prior to their marriage.
    So, if I understand correctly:
    A and B are full second cousins once removed. (2C1R)
    You and A are half or full 2C1R
    You and B are half or full 2C.

    DNA painter tool says both options are possible.

    You can be 2C or half-2C with B.
    You can be 2C1R or half-2C1R with A.
    Also, A and B can be full or half-2C1R.

    Try the tool yourself:
    https://dnapainter.com/tools/sharedcmv4

    Keep positive, dna may still help you solve the mystery:

    If you happen to match a third cousin or two on the paternal side of your grandmother, you will know that mother's husband is very likely the father. (Or one of his relatives, to keep in mind all the theoretical possibilities.)

    If, instead of that, you discover a close match to you, from an unknown family, who only matches you (and possibly other descendants of your grandmother) ... that could be the link to grandmother's unknown father.

    Anyone here with a better idea?

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    • #3
      I second all that Emona's said. And testing descendants of your (theoretical) greatgrandfather's siblings, if you can, should be very helpful.

      There's another tool at dnapainter that could also help: the What Are the Odds (WATO) tool. You need to register with dnapainter and be signed-in to use it. It's in dnapainter's tools section; it's also easily google-able. You could have a play with it now, with just the two matches - but as your two sets of cMs are so 'ambiguous' the tool probably won't be much more helpful than working it out manually. Adding more matches, if you find them, would give the tool more to work with.

      There's info about the WATO tool at http://thednageek.com/science-the-he...ur-dna-part-7/ There's also a facebook group for users, if you happen to be on facebook.

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      • #4
        It is unclear to me the suspected relationship A and B are to you or each of their relationship is to each brother. This is needed to give you accurate info

        Comment


        • #5
          Thank you all for taking the time to help me. My grandmother was the older sister of two brothers but she was born before the brothers and a couple of years prior to the marriage of the parents of the two brothers.
          B and I are either full or half second cousins.
          B is the grandson of my grandmother's full or half brother.
          A and I are second cousins, or half second cousins, once removed.
          She is the great grandaughter of the other brother.
          A and B 's mothers are daughters of the two brothers. They have a 60cM X match together. I have no X match. The descent from my grandmother was through my father.

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