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When an autosomal match is not a match

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  • When an autosomal match is not a match

    Have you ever been sent an email inquiry about an autosomal match and after reviewing your tree and all the possibilities, you just come to the conclusion with the other person that either a. the match is just a random DNA segment match...we aren't actually related or b. we may be related but it's probably not in our paper trails so it's kind of a meaningless match? I've had a couple inquiries recently where I've just come to the concusion that this person couldn't possibly be related to me anywhere within a 5th cousin range or if they are, it's so far back, there's no way to make a meaningful connection.

  • #2
    The answer depends on the strength of the match -- the size of the matching segment in cM. If it's over 10, there's probably something there. If it's over 15, it's almost certainly real. However, the actual relationship could be quite a ways back. Autosomal DNA is transmitted in about 3 dozen chunks, as a result of recombination and the random segregation of chromosomes at each generation, so it is not the case that everything is completely scrambled. Rather, we expect some segments to be transmitted more or less intact through several generations simply by chance.

    A couple rules of thumb bear on this problem: roughly 10 percent of actual 3rd cousins don't match each other by the usual default autosomal matching criteria, and about 50 percent of 4th cousins likewise don't match. So by the time we get out to your 4th cousins, quite a few of them won't have enough shared DNA to match you, even though some of them will match each other. But if many 4th cousins end up with too little shared DNA to match you, there must also be a lot of other 4th cousins who got MORE than the theoretical share of matching autosomal DNA. Some 4th cousins, and indeed some 5th, 6th, or even 7th cousins apparently can be identified among our matches. I've found some 4th cousins who share only about 9 cM with my father, but there are enough individuals involved, and a clear enough paper trail to convince us that we are on the right track to identify our mysterious McDougall ancestor.

    In most cases, it will be very difficult or perhaps impossible to prove the identity the common ancestor when the connection is so far back, because few of us have complete pedigrees, and any of our pedigrees could potentially contain one or more errors. If, as happened with my McDougall family, there are multiple people who descend from different branches of both families, and enough of share the same segments, and if the pedigrees don't overlap in any other way, there may be enough evidence to lead to a solution..

    When I get an inquiry about a single segment match, I look for an obvious connection in the family history, and I look for triangulation with other known relatives. Usually I don't find anything, and respond back that, while there is probably a distant relationship, we don't have enough evidence yet to reach a more definite conclusion. I'm afraid most single-segment matches are like that, but we keep trying, because every so often one of them can be identified.

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    • #3
      It depends how much DNA we share and whether we have any matches in common that I've identified. I had one person at AncestryDNA decline to give me an invite to her tree because she didn't think we were closely related enough to bother trying to figure it out (even though we share 24 cM in total across one segment). Years went by, and I finally identified the same MRCA of 3 matches I had in common with her. This helped me break down a brick wall in my tree and find the right records to confirm it. I message her to let her know that 4 her of matches are descended from this same MRCA so that's probably the branch we match on as well.

      According to ISOGG, 15+ cM is supposed to have a 100% chance of being identical by descent. 11 cM has a 90% chance of being IBD. https://isogg.org/wiki/Identical_by_...sitive_matches

      So I would definitely continue to pursue anything 15+ cM, and probably anything over 11 cM too (segment length, not necessarily total).

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      • #4
        For me, one guy had 13.43 CM segment and one part of his family was from the same area as the suspected side, but we didn't connect on any names. I had to tell him that either it's a random match or we match probably somewhere in the 1600's...past our paper trails.

        The other person, I was able to use DNA painter to find the exact DNA segment we were matching on - it was a 20.96 CM segment...an ancestor/wife born in the 1730's. But once again, none of his furthest ancestors matched anyone in my umbrella..like I mentioned before, I can pick up anywhere in a 5th cousin range with a furthest ancestor. I also didn't match that person's Y-numbers. So I said it was probably either one of the wives going back from the 1730's or it was a descendant of a brother or sister from my paternal line from off my paper trail. The person never responded back, probably because it was useless.

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