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  • Methodology Question

    I think I'm starting to think myself in circles trying to interpret DNA results at this point!

    So I have my mother's autosomal results, her third cousin's and a second cousin's, both from her paternal side/line.
    I downloaded all 3 of their chromosome browser results into Excel files, then combined them and sorted them according to Chromosome and start location...
    There's a few spots where all 3 match up/share the same segment, for example on chromosome 5 from 138196426 to 141532929. Is it safe to estimate that other matches who share that similar segment might also share a common ancestor with the three, even if they only show up as a match to one or two?

    Taking that a step further, another lady who is a match to 2/3 of the known related people shared her chromosome browser results with me, and I also added to this massive chromosome spreadsheet.
    She has a match that matches her on chromosome 5 from the same Starr and end positions as the 3 above share. Does this mean this match of hers could also share an ancestor with the 3 cousin's above?? Or is it a coincidence that she has a match on the exact same location from a different line in her family??

    Am I thinking too hard? Haha.

  • #2
    You can help us help you by uploading your kits to GedMatch and telling us the kit numbers.

    Regarding what you have told us, your chromosome 5 segment represents only 1.6 cM for males and 5.2 cM for females (around 3.5 cM sex-averaged). That is not a strong match and alone would not be one I would spend a lot of energy on. You didn't tell us whether the data was build 36 or 37, but the answer doesn't change much either way.

    GedMatch will do all those comparisons for you/us across all the chromosomes, including the X. Donate a few bucks and you can try their advanced tools, too.

    Bob H.

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    • #3
      Gedmatch appears to be down at the moment
      kit numbers F362048 and F371429 are my mom and the third cousin, the second cousin tested with ancestry and I'll have to check his number when gedmatch is back up, I didn't memorize it yet. ..
      the segment isn't impressive in size. But it is exact. Looking at other matches of my mother's who I know are related, to each other closely, they share exact segments down to 1 or 2 cM...
      I should mention the family in question here is from rural Latvia, and we don't have many close matches like people of Ashkenazis or British Isles, etc.
      Besides our purposely-tested, known family, my mom's closest match is only ~18 cM.

      Comment


      • #4
        I see there are only around 20 people who share 7 or more cM with both and only one that shows multiple segments with both [A933943]. I see why you are focusing on the small segments as that seems to be what you have to work with. I understand your situation and your question better now.

        As you are likely aware, there are 2 sides to the debate over validity of small segments. I personally believe they must be valid but very hard to do anything with. Admixture calculators draw significance from small 'runs' of SNPs and as we know having a given single SNP can be of significance, so to say they are patently not useful is hogwash.

        I think your suggested use as a hint of a regional location of common ancestors somewhere in the past is valid and reasonable. The DNA was inherited from somewhere... Until you get closer-in matches I'd keep plugging at finding a thread. Keep us informed of any revelations or techniques you might discover. Your spreadsheet seems to be the best tool you have, though it will get very unwieldy. If your techniques show merit, maybe one of the resident IT gurus may code something simpler to use.

        I expect folks may jump all over this thread; don't expect me to engage in defense of my opinion as I lack the energy to waste...

        Bob H.

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        • #5
          Close your eyes, then step outside.

          If you feel a droplet on your hand, you would not know whether it starts (or ends) raining, or the droplet came from a sprinkler or it came from the passing car spraying the windshield.

          If you open your eyes, and it is raining, but you are close to a sprinkler, you still would not know where a particular droplet came from.

          W. (Mr.)

          P.S.
          So to answer your question
          Is it safe to estimate
          No it is not safe, as the segment is too small. Although, as Bob has pointed out, it depends on sex. For males, that common ancestor can be quite far. How endogamous your mother population is?

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          • #6
            Thanks for the responses guys .
            Mr W, my mom doesn't appear to have any close endogamy (surprising, I expected it actually due to the small rural farming communities her people hailed from) however both the third cousin and the fourth lady who matches the known family both appear to have Ashkenazi lines and that's definitely reflective in large groups and pileups of matches on certain segments.
            I had a feeling I might have been trying to make mountains of molehills! It's a population of young surnames and lots of holes in the paper trail records, I've got my work cut out!

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            • #7
              GedMatch is back up.

              Attached are a couple of files; maybe you have done this already and if so, I apologize.

              Kits F362048, F371429, and A449628 are clearly related to each other, sharing multiple [5 to 17] segments of good size[47.5 cM at least], each to the other individually]. However they all three do not share any segments 5 cM or greater. I see why your [and now my...] head hurts... I'm sure it will make sense if you could put the relationship on paper but you must realize a common ancestor to all 3 might not be in the cards... Because each pair has a common ancestor does not guarantee all 3 share one anytime recent.

              The only meaningful way to get the matching segment printout to you was to zip it. Enjoy; I prefer ibuprofen...

              Bob H.
              Attached Files

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              • #8
                Originally posted by rachelleleclaire View Post
                Thanks for the responses guys .
                Mr W, my mom doesn't appear to have any close endogamy (surprising, I expected it actually due to the small rural farming communities her people hailed from) however both the third cousin and the fourth lady who matches the known family both appear to have Ashkenazi lines and that's definitely reflective in large groups and pileups of matches on certain segments. [----]
                Hmm, small rural community is like Ashkenazim. It does not need to be close engogamy, but I can see the endogamy through centuries of marriages within the same, small gene pool.

                W. (Mr.)

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                • #9
                  So here's a little chart I made explaining the known paper trail connections between the 3, that might explain what could look like endogamy..
                  A brother and sister married another brother and sister. So the second cousin actually shares the same 4/8 great grandparents with his first cousins...
                  Attached Files

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