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Significance of female x-matching

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  • Significance of female x-matching

    OK- if two females are X-matches , how does that help as neither will know whether the matching X is maternal or paternal in origin?

  • #2
    Originally posted by sharavrose View Post
    OK- if two females are X-matches , how does that help as neither will know whether the matching X is maternal or paternal in origin?
    You can put a big X through any lines which have two males in succession.



    • #3
      As Jack posted, with each generation back from the two sides of the match, more ancestors are ruled out as the source of the shared x chromosome segment. This is because of the unique inheritance pattern of the x - male ancestors in two succeeding generations (father and son) can't pass on any DNA from the grandparents' x chromosomes. That makes it easy to eliminate which ancestors even have a chance to be the source of a shared segment on their descendants' x chromsome(s).

      It's easier to understand by looking at this diagram from Blaine Bettinger showing x chromosome inheritance for a female - Only the ancestors in pink (female) or blue (male) boxes contribute DNA to the x chromosomes in the female descendant at the center of the diagram. Any ancestor in a white box has nothing to do with any DNA segments on the x chromosomes of the female descendant for this diagram.

      So, you can print out the diagram twice and fill in the names of your ancestors and the ancestors of your match who shares an x chromosome segment with you. It won't tell you whether the common ancestors are on the maternal or paternal side. But it will eliminate entire lines of ancestry from having the common ancestors. For instance, right away you can eliminate the paternal grandfather's ancestry of you and your match as containing the common ancestors, since neither of you inherited any x chromosome DNA from your paternal grandfathers. Also, as you fill in the white boxes in the diagram, you can also eliminate the ancestors in those boxes and their lines of ancestry. If you and your match have family trees that are documented back a good number of generations, that will help you eliminate many lines of ancestry for the common ancestors for the segment.


      • #4
        right off the bat, you can eliminate your paternal grandfather's line. So that rules out 25% of your family and 25% of hers.

        After that, it gets a little trickier and you should print out those charts that MMaddi refers to. But you can eliminate a few more combinations in every generation.


        • #5
          Be cautious, though, because people can be related on more than one line. One example of mine:

          Me to match (female to female), 40 cM on one chromosome, 5.7 cM on the X

          Phased paternal kit (based on my mother's DNA) - 40 cM, and the X disappears

          Mother's kit (not phased) to match - 5 cM, and 9.9 cM on the X

          It's clear that the match and I have shared matches on the paternal and maternal side, although my parents do not have any recent shared ancestry or geography.

          So all you can say for sure is that the X match cannot be through two males. But that does not mean the autosomal match is the same line as the X match, although of course it is likely.


          • #6
            One thing nobody ever seems to mention is the fact that part of the X chromosome recombines with the Y Chromosome.

            So it is in fact possible for you to inherit part of your X chromosome from your mother's father's father. Your mother's father's X partially recombined with his Y, so one of your mother's X's contains a portion of her father's Y, and then her two X's recombine and you likely get some of that passed onto you.

            The same holds true for your father. Part of his X and Y recombined, so if you are male, you received part of the X he got from his mother in your Y chromosome, and of course the rest of the Y from him, but even his Y was a combination of his father's X and Y. If you are female, you still receive part of the Y that your father got from his father.

            Moral of the story is, the X match doesn't filter out the part of the X that recombines with the Y, so it is not a very accurate indicator of anything.

            Oh, and here's a video that shows how the X/Y recombination is different than the other chromosomes!