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Not a match at FTDNA

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  • Not a match at FTDNA

    I'm re-posting this - a simplified version of an earlier post (August 7) which sank into oblivion

    Many thanks if someone can help with an answer.

    I have a match on Gedmatch at 20.5cM total & longest segment 13.6cM (using the one-to-many tool). We both tested at FTDNA but don’t show in each other's matches there ... even though the Gedmatch data suggests we meet the threshold for FTDNA matches. And there are smaller total and longest-segment matches in my matches lists at FTDNA.

    A one-to-one comparison of our two kits at Gedmatch shows the 13.6cM segment as valid.

    The lack of a match at FTDNA doesn't seem to be related to our Settings - we've experimented with these and it's made no difference.

    Is it likely/possible that FTDNA and Gedmatch are reading the dna sequences differently, so that FTDNA has broken the the 13.6cM into two or three smaller segments?

    Yes, it's a small segment. But the match is part of a wider puzzle. And I've become interested in how/why this can happen.

    Again, my thanks to anyone who can help

  • #2
    I seem to recall that FTDNA has an algorithm that doesn't take into account smaller cM matches, thus affecting the segment total and overall cM count, which obviously forces some matches to not show up.

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    • #3
      Thanks, spruithean ... although I'd have thought there'd still be a match under the "one segment >9cM, regardless of total cM" criterion. Which is why I wondered whether FTDNA sees the 13.6cm as smaller separate segments

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      • #4
        Oops, I responded to your older post and then saw this. I'll recopy that response here:

        When you say you don't appear on each other's match lists, do you mean that when you search his surname with the search function, he doesn't appear? There is a known bug with the search function that this sometimes happens with weak matches. (But I have yet to see an FTDNA representative state this or indicate whether there are plans to fix it).

        He may appear if you download your list of matches from the match page or the chromosome browser page and search for him in the spreadsheet, or if you search for him in the list of matches when using the matrix function accessible from your homepage. At least, my weak matches who are missed by the search function still show up in these places.

        The workaround for the search function is only to type the first few letters of his surname in. Jim Bartlett suggested that, and it works for me.

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        • #5
          I've seen a similar situation with two siblings whose kits I manage. Back in May a new cousin appeared as a match at FtDNA to one of them. I waited a couple of days, but they still didn't appear as a match to the other.

          They're a 3rd cousin to both of them. Today he appeared as a new match to both at Gedmatch. According to Gedmatch he shares 47cM with one of them and 17cM with the other. He also shares 143cM with a 3rd cousin from a different gt-grandfather (so another 3rd cousin), and 140cm with a man whose gt-grandmother was the sister of the other gt-grandfathers (so also a 3rd cousin).

          The way Gedmatch and FtDNA add the shared DNA differs, and sometimes this means people don't appear on your match list. I've seen this with Ancestry matches as well.

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          • #6
            Hi barryrsmith ... thanks for your suggestions

            His is a very common surname and appears in many trees and/or surname lists. When I use Search I find about four pages of results (including others I can't find under their own surname). I've also laboriously searched for him in the lower levels of my match pages, sorted first by total and then by longest shared cMs. He's not there. Nor am I in his matches.

            I hadn't thought of the Matrix and the csv/spreadsheet. I've just done that and he's not there either.

            I'm about to open a request, to see if FTDNA can explain why. Fingers crossed!

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            • #7
              Originally posted by ltd-jean-pull View Post
              ... The way Gedmatch and FtDNA add the shared DNA differs, and sometimes this means people don't appear on your match list.
              Thanks, ltd-jean-pull - it's good(?) to know I'm not alone!

              Ta also for your comment that Gedmatch and FTDNA add the shared dna differently. As I mentioned above, I'm about to open a request about this - it'll be very interesting to see what FTDNA says ... and hopefully I'll be able to understand the answer

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              • #8
                The Answer

                Originally posted by Fern View Post
                I'm about to open a request, to see if FTDNA can explain why.
                The reply arrived in under 24 hours It answered my question, and in a way I could understand:

                "GEDMatch examines different segments from us, and has different thresholds for matching. By way of example to illustrate this, if they look at a chromosome and look at the segment comprised of markers 100-200, and we look at the same chromosome and look at the segment comprised of markers 1-155, and you only match on markers 154-200, then you would look like a non-match to us, and a pretty solid match at GEDMatch. To try to avoid this confusion, we look at over 700,000 markers in your autosomal DNA, to try to get the most thorough possible coverage, but sometimes it just so happens that the markers looked at by other companies tell a different story than the ones that we look at. (bolding added by Fern)
                Last edited by Fern; 5 September 2018, 04:46 PM. Reason: clarify use of bold type

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