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Significance of 41 cM segment on X chromosome

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  • Significance of 41 cM segment on X chromosome

    I have an estimated 3rd cousin match with a total of 106 cMs shared. This total includes a 41 cM segment on the X chromosome.

    47000000 114000000 41.6 cM 4246

    The x chromosome was inherited from her paternal grandmother (Ashkenazi). I am adopted so having a single line to research is very helpful. Can anyone estimate the number of generations back I need to target my research?

    Thanks in advance,
    Brenda Ramsey

  • #2
    Originally posted by bramsey View Post
    I have an estimated 3rd cousin match with a total of 106 cMs shared. This total includes a 41 cM segment on the X chromosome.

    47000000 114000000 41.6 cM 4246

    The x chromosome was inherited from her paternal grandmother (Ashkenazi). I am adopted so having a single line to research is very helpful. Can anyone estimate the number of generations back I need to target my research?

    Thanks in advance,
    Brenda Ramsey
    The answer to your question, which I've bolded, is "it depends." You have two factors that indicate that the relationship is significantly further back than the estimated 3rd cousin level.

    First off, in general because of the unique inheritance pattern of the x chromosome, a shared segment can survive recombination over many more generations than segments on chromosomes 1-22. Only certain ancestors can possibly pass along DNA to their descendants' x chromosome(s). Here are two useful diagrams from genetic genealogist Blaine Bettinger to illustrate this - http://www.thegeneticgenealogist.com...8/12/1male.png (for males) and http://www.thegeneticgenealogist.com...2008/12/1b.png (for females). (Only descendants in a blue or pink box pass on any DNA to the descendants shown in the diagrams.) Immediately, you can see that none of the ancestors of males on the paternal side contribute DNA to the x chromosome of their male descendants. Even for females, many of the ancestors on both the paternal and maternal sides don't contribute any DNA to the x chromosomes of their female descendants. This means the x chromosome goes through less recombination in each generation than chromosomes 1-22 and segments last longer.

    The other factor is that you state that your match received one of her x chromosomes from her paternal grandmother, who was Ashkenazi. I'm not sure how you know for sure that the x on the match is on the x she received from her father (which came from his mother), since she has two x chromosomes, one from each parent. But if it's the case that the shared segment came from her Ashkenazi paternal grandmother, then the centuries of intermarriage among Ashkenazi Jews means that this segment could be very common and passed back and forth across many generations. That would also tend to make the segment last longer and represent a more distant ancestor than a 3rd cousin estimate would indicate.

    The key question is how long are the other segments you share with this match. You mention that you share a total of 106 cM with her, which includes the 41.6 cM x segment. How large are the next two or three largest segments? If she is anything like a true 3rd or 4th cousin, I would think that the next two or three largest segments should be about be at least double digit in cMs.

    As an example, I have a match who is a known paper trail 3rd cousin, once removed. FTDNA's chromosome browser shows that we share segments of 19.81 cM and 14.86 cM. The next largest segment is 6.32 cM. All the other shared segments are less than 5.0 cM, a level most genetic genealogists would not consider as necessarily coming from a common ancestor.

    So, if you share at least a couple of segments on chromosomes 1-22 that are double digits, then the match may be as close as a 3rd to 5th cousin. If not, I think you probably are dealing with a fairly distant cousin, which looks closer due to the x chromosome shared segment, coming from an Ashkenazi line.

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    • #3
      My match's father was Ashkenazi and her mother is Scots-Irish. Since I am half Ashkenazi and have no discernible Scots-Irish ethnicity in any of my three atDNA test results, it is reasonable to assume that we are matching on her father's X chromosome.

      All shared segments:
      chr 6 - 5.5 cM
      chr 6 - 27.6 cM
      chr 16 - 16.3 cM
      chr 18 - 15.0 cM
      chr X - 41.6 cM

      Comment


      • #4
        Originally posted by bramsey View Post
        My match's father was Ashkenazi and her mother is Scots-Irish. Since I am half Ashkenazi and have no discernible Scots-Irish ethnicity in any of my three atDNA test results, it is reasonable to assume that we are matching on her father's X chromosome.
        Yes, that sounds reasonable to me.

        Originally posted by bramsey View Post
        All shared segments:
        chr 6 - 5.5 cM
        chr 6 - 27.6 cM
        chr 16 - 16.3 cM
        chr 18 - 15.0 cM
        chr X - 41.6 cM
        That looks good for a fairly close cousin relationship, something like a 3rd cousin. However, the fact that the common ancestor is through an Ashkenazi line may still be making the relationship look closer than it actually is.

        I think that FTDNA adjusts their algorithm for estimating relationships when Ashkenazi ancestry is involved. I know that 23andMe does make an adjustment.

        I'm not familiar enough with Ashkenazi DNA matching to make a good judgement on how closely you and the match are related. Maybe someone with more direct experience with this, like josh w or khazaria (frequent posters here), can advise you about how close the relationship seems to be, based on the size and number of shared segments and their knowledge of how Ashkenazi ancestry affects that.

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        • #5
          Thank you for your insight.

          Brenda Ramsey

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