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Genetic distance of 2 at Y-DNA67 but surnames are different query

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  • Genetic distance of 2 at Y-DNA67 but surnames are different query

    If a man appears on my list of matches (with same haplogroup) for surname e.g. "Smith" and a genetic distance of only 2 at level Y-DNA67 but with a completely different surname e.g. "Brown" could this suggest that my male line "Smith" are descended from a "Brown" or vice versa? Or could such a match occur by coincidence at this level? I suspect a Big Y test would clarify the position.

  • #2
    If that's the only close match you have, I don't think you can draw any conclusions at all, unless you have a strong family tradition that your family surname was "Brown" at one time. It could even be that Brown is the name of the person who paid for the test and the test-taker is really a Smith. Or a person could have changed his name for many reasons.

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    • #3
      Some men happen to land in a cluster where almost everybody has the same surname. That really does happen, especially if you are a Buchanan, or a descendant of Deacon Edmund Rice of New England. However, there are at least as many clusters of matches for Y-67 where there are many surnames within the cluster. A plausible explanation is that the defining mutations for such clusters happened well before stable surnames came into use, so that men with the same Y chromosome genetic background ended up eventually with diverse surnames.

      The Big Y test can definitely shed some light on the problem. For me, the Big Y showed that I am out more or less by myself on a little twig on the Y haplotree, possibly near where the McMillan clan branched off, and way, way far removed from all the other McCoy families that have been tested. That result doesn't tell me who my ancestor was (no really close matches), but it gives me a strong reason to ignore all of the other McCoy families who have been tested -- they are very clearly not mine!

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      • #4
        Originally posted by Willhay99 View Post
        If a man appears on my list of matches (with same haplogroup) for surname e.g. "Smith" and a genetic distance of only 2 at level Y-DNA67 but with a completely different surname e.g. "Brown" could this suggest that my male line "Smith" are descended from a "Brown" or vice versa? Or could such a match occur by coincidence at this level? I suspect a Big Y test would clarify the position.
        A GD of 2 at 67 markers has a 95% chance of sharing a MRCA within 14 generations, so there is a chance that your MRCA simply predates the development of your surnames, so your surnames developed independently. If you click on the TiP icon for each match, it will give you more specific probabilities. However, my dad's closest Y match had a GD of 4 at only 37 markers and their bio surnames are the same and this actually helped me find my grandfather's unknown father.

        Assuming the surnames aren't actually as common as Smith and Brown, what do you find for those surnames among your autosomal DNA matches? This is what I did - I took the Y match surname (which was not common) and searched for it among my autosomal matches and found that everyone with that surname in their tree descended from the same man. If you're finding this as well for "Brown" then there may have been a NPE in your male line somewhere and you may actually be descended from "Brown". If the surname is common, you may not have as much luck doing this.

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        • #5
          I have a 0 genetic distance with a person with a Y-dna test to 67 markers. I do not show up as any match with him on an Ancestry autosomal test. I have explained this to myself as follows:

          FT-DNA is a Y-chromosome test (outside of Family Finder). If the Y-chromosome is passed unchanged through the male line, I could theoretically have the same Y-chromosome as a very distant cousin if we inherited it through our respective male lines from a distant relative. One would expect the surname to be the same unless there was at least one adoption, illegitimacy, etc. or unless it went back far enough that the surnames changed as in patronymics or variant spelling.

          The autosomal DNA in such a case becomes diluted over time such that we might have nothing in common any more. If each generation cuts the amount of shared autosomal DNA in half, in 5 descending generations the shared DNA is only 1/32 of the original parent. (This example is within the same family. If we are comparing two separate families, the shared DNA would be diluted twice as fast, I think.) With recombination, there might be nothing shared at that level. I predict that there will also be no match in the Family Finder data.

          Is this the correct way to look at my situation?
          Thanks for any help you can give.

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          • #6
            Originally posted by crich8136 View Post
            I have a 0 genetic distance with a person with a Y-dna test to 67 markers. I do not show up as any match with him on an Ancestry autosomal test. I have explained this to myself as follows:

            FT-DNA is a Y-chromosome test (outside of Family Finder). If the Y-chromosome is passed unchanged through the male line, I could theoretically have the same Y-chromosome as a very distant cousin if we inherited it through our respective male lines from a distant relative. One would expect the surname to be the same unless there was at least one adoption, illegitimacy, etc. or unless it went back far enough that the surnames changed as in patronymics or variant spelling.

            The autosomal DNA in such a case becomes diluted over time such that we might have nothing in common any more. If each generation cuts the amount of shared autosomal DNA in half, in 5 descending generations the shared DNA is only 1/32 of the original parent. (This example is within the same family. If we are comparing two separate families, the shared DNA would be diluted twice as fast, I think.) With recombination, there might be nothing shared at that level. I predict that there will also be no match in the Family Finder data.

            Is this the correct way to look at my situation?
            Thanks for any help you can give.
            Yes, even 3rd cousins have a small chance of not sharing any detectable autosomal DNA. A GD of 0 at 67 markers has about a 95% chance of sharing a MRCA within 7 generations - that's a 5th great grandfather, meaning around 6th cousins or closer with other testers.

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