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  • X Chromosome segment only

    Hi, maybe a silly question, but on running my raw data through GedMatch, I see a few matches in their database where I only match on the X chromosome and no where else, and some of the segments are quite lengthy, >20 cM

    Can someone explain what this means to only match on the X?

    Does it mean any different to only matching in one place elsewhere?

    Thanks

  • #2
    Originally posted by Seth_Rogoff View Post
    Hi, maybe a silly question, but on running my raw data through GedMatch, I see a few matches in their database where I only match on the X chromosome and no where else, and some of the segments are quite lengthy, >20 cM

    Can someone explain what this means to only match on the X?

    Does it mean any different to only matching in one place elsewhere?

    Thanks
    I would call a single segment over 20 cM genealogically significant especially since you are a male and you only have one X. That lowers the chance of a false match.

    A 20 cM segment can be coming from a common ancestor farther back in time if it is on the X chromosome when compared to an autosome. The range of potential number of generations back to common ancestor is quite large because a segment does not shorten at all when passing through a male. The more males in the pathway (father to daughter to father to daughter etc.) means that the common ancestor would likely be farther back in time. There cannot be two males in a row. There have to be women in the line of descent which means you both need to have a great pedigree chart with lots of surnames to find the match.

    If you have no autosomal matches among these X matching cousins, then you know you are unlikely to be a close relative. So I would guess that the cousinship would likely be beyond 3rd cousin up to quite distant cousins.

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    • #3
      Thanks for the reply

      Comment


      • #4
        Originally posted by Seth_Rogoff View Post
        Hi, maybe a silly question, but on running my raw data through GedMatch, I see a few matches in their database where I only match on the X chromosome and no where else, and some of the segments are quite lengthy, >20 cM

        Can someone explain what this means to only match on the X?

        Does it mean any different to only matching in one place elsewhere?

        Thanks
        The presence of X DNA for a male means the match must be on your mother's side. The presence of only X DNA doesn't mean anything though, it's no different than when you have a match only on chromo 1, 2, 3, etc. It still means the match is from your mother's side but there's no difference between sharing only X DNA and sharing X DNA plus DNA on another chromo. I know it seems like sharing X DNA but no DNA on any other chromo should be significant, but it's not.

        Comment


        • #5
          To go with that, if you consider it, a closer cousin match, like a 2nd cousin, would probably have autosomal matches too (in conjunction with this X share you mention), while a more distant one might not necessarily.

          So the point is, if you see just one large segment shared, like chromosome 3, or 5, or the X, it is probably a more distant ancestral connection in comparison to a share made up of all three at the same time; chromosomes 3 and 5 and X.

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          • #6
            Thanks again for all the replies

            Comment


            • #7
              X match only but with a wrinkle

              Per the thread and as a male, I understand that my X chromosome comes from my mother and therefore any matches must be on that side of my family. I have an additional question about two of my X matches, however, noting that we have no other matching segments above the 7 cM threshold.

              Match #1 is a female:

              Chr Start Location End Location Centimorgans (cM) SNPs
              X 116771182 135657508 23.5 2233


              and match #2 is a male under the control of the above female:

              Chr Start Location End Location Centimorgans (cM) SNPs
              X 116771182 135519957 23.5 2237

              Without knowing the relationship of the two matches, can I infer anything from the above?

              Thanks

              Comment


              • #8
                Originally posted by Haggis View Post
                Per the thread and as a male, I understand that my X chromosome comes from my mother and therefore any matches must be on that side of my family. I have an additional question about two of my X matches, however, noting that we have no other matching segments above the 7 cM threshold.

                Match #1 is a female:

                Chr Start Location End Location Centimorgans (cM) SNPs
                X 116771182 135657508 23.5 2233


                and match #2 is a male under the control of the above female:

                Chr Start Location End Location Centimorgans (cM) SNPs
                X 116771182 135519957 23.5 2237

                Without knowing the relationship of the two matches, can I infer anything from the above?

                Thanks
                My best guess is that these two people (who match you) are closely related, perhaps father and daughter or mother and son. I would also expect the segment to be robust if it is coming from a male. That means less chance of a pseudo-segment, or false positive. Other than that you can't infer anything except that you would want to follow the X pattern of inheritance back in time. If you are a male and this X is coming from a male, you can limit the number of ancestral lines quite a bit. You will likely have to go back beyond great-great-grandparents and it is somewhat unpredictable but you probably can find this match within a genealogical time period if you have a strong pedigree. Just skip any ancestor or his forebears where there are two males in a row (that means father to son transmission).

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