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Proving or disproving a remote autosomal match

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  • Proving or disproving a remote autosomal match

    I've heard of things like this happening before, so I figure somebody here will be able to help me figure out this match.

    I have what FTDNA estimates as a fifth to remote cousin, a male with a rare surname of French origin, whom I have 26 matches in common with : yet he is not a match to either of my parents, nor my two siblings who have tested. We are said to share 21 centimorgans. According to the chromosome browser's detailed segment data, we share
    2.68 centimorgans on chromosome one,
    2.14 on chromosome two,
    1.24 on chromosome three,
    1.92 on chromosome eight,
    7.89 on chromosome nine,
    1.82 on chromosome ten,
    1.54 on chromosome eleven,
    and 2.16 on chromosome fourteen.
    Our SNPs go along the lines of
    1,100 on chromosome one,
    600 on chromosome two,
    800 on chromosome three,
    596 on chromosome eight,
    1,000 on chromosome nine,
    500 on chromosome ten,
    800 on chromosome eleven,
    and 500 on chromosome fourteen.

    One of our common matches is a second to fourth cousin, five are third to fifth cousins, nine are fourth to remote cousins, and the remaining eleven are fifth to remote cousins like he is. Twelve of them are maternal matches, five paternal and I haven't checked to see if the others are in common with one or the other of my parents yet. Three of them have surnames in their family trees in common with mine, two of them being maternal matches and the names are on the maternal side of my family tree, even the match that hasn't been grouped as maternal or paternal yet has a name that is on my mother's side. Most of them are ancestrally very British Isles, as FTDNA puts it, or German, but a few of them have French ancestry. I understand most people disregard the My Origins feature, but I checked this match there and found that the only ethnicity we are estimated to have in common is Western European, 87 per cent for him and 42 per cent for me. It is interesting to note that my mother is only four per cent Western European and my father shows none at all, which I know most people would disregard as inaccurate but I don't question since for that matter people can inherit physical features from their grandparents that their parents don't.

    Anyways, to make a long story short, I'm just asking, do I actually have a common ancestor with this man, or is it just an obscure and misread DNA result? Than you all for your assistance in these matters, DNA and genealogy can be tricky sometimes, so I thought I'd ask, especially what with the low centimorgans on the chromosome detailed segment data page.

  • #2
    The only segment I would regard as a match for purposes of genealogy is the one on chromosome 9. If it were me, I would want to see how each of the kits you have identified match each other, segment by segment. I would only deal with confirmed triangulation groups. I would want to identify all the kits that match on that same segment on chromosome 9, and I would want at least one from the set that match on that same chromosome 9 segment to have a sufficiently detailed pedigree to suggest that there is sufficient reason to believe that you have a common ancestor. Without that level of specific information, I would regard the estimated degrees of cousinship as too vague to be of much use. In general, I have found the FTDNA estimates of relatedness to be extremely optimistic.

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    • #3
      Thank you for your reply, I'll check that when I get the chance. Sorry to sound so ignorant, but what is confirmed triangulation?

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      • #4
        Triangulation means that you have actually checked each kit against every other kit in the "triangulation group" and have confirmed that they actually match each other on the SAME segment in all combinations. If you have control of most of the kits, or if your matches can cooperate, you may be able to accomplish that here on FTDNA. GEDmatch is a good alternative, you will be able to compare every kit with every other kit regardless of who "owns" it.

        One might question the SNP counts for the short segments that you described in your post. I don't think FTDNA actually tests for that many SNP's! Has FTDNA started using "imputation", where a statistical decision is made that if some SNP's match in a given segment, there is a high probability that other SNP's in the same region match as well, even though they are not tested? I'm not a fan of imputation, at least when it used to create artificially inflated SNP counts that make a segment look far more reliable than it should be. I hope someone can chime in here to tell us what's going on.

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        • #5
          Originally posted by John McCoy View Post
          Triangulation means that you have actually checked each kit against every other kit in the "triangulation group" and have confirmed that they actually match each other on the SAME segment in all combinations. If you have control of most of the kits, or if your matches can cooperate, you may be able to accomplish that here on FTDNA. GEDmatch is a good alternative, you will be able to compare every kit with every other kit regardless of who "owns" it.

          One might question the SNP counts for the short segments that you described in your post. I don't think FTDNA actually tests for that many SNP's! Has FTDNA started using "imputation", where a statistical decision is made that if some SNP's match in a given segment, there is a high probability that other SNP's in the same region match as well, even though they are not tested? I'm not a fan of imputation, at least when it used to create artificially inflated SNP counts that make a segment look far more reliable than it should be. I hope someone can chime in here to tell us what's going on.

          I haven't contacted any of my matches to check, but I know that my match only shares enough DNA with me to really count on the ninth chromosome, and of the 26 matches we have in common, in chromosome browser only two of those match me about the same as him on the ninth chromosome. Not that that means anything, but I find it interesting.
          Last edited by Rhonda Hatton; 6 July 2020, 07:27 PM.

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