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Help understanding X match

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  • Help understanding X match

    I have been trying for the past two years to understand X matching; every time I think I've got it, something comes up to confuse me. I have two scenarios I would like to have clarified.
    I have two family finder matches who are brother and sister. I share 15+ cM on the X with the sister but am not an X match with the brother. So does this mean the X I share with the sister had to have come from her father and not her mother?
    With my own siblings, I have three sisters who have had their DNA done and since we each got half from our mother and half from our father, I assume our X is exactly the same. So how is it possible for one of us to match someone on the X (at a fairly significant level, I might add - almost 50 cM) and another sister not match that person on the X?

  • #2
    First scenario: you match as a distant relative (?) to a sister and brother. The brother got his X from his mother, who had two X chromosomes, and none from his father (because the brother is a male). He got some combination of those two X's, of which one segment of 15 cM happens to match you. The sister got one X from her father (who only had one to pass on), and one X from her mother, who had two X chromosomes. Whatever combination of segments from her mother that she got, it did not happen to include the segment that matches you, that her brother got. So, it's all a result of the recombination and the random assortment of chromosomes.

    Second scenario: What do full siblings get from their parents? All the daughters get the X chromosome from their shared father, plus an X chromosome derived by recombination from their mother's two X chromosomes. As a result, all the full sisters will have AT LEAST a "half-identical region" spanning the entire length of the X chromosome (about 195 cM) PLUS, very likely, some additional matching segments (shown as "fully identical regions" in some chromosome browsers, but not here on FTDNA). All the sons get only one X chromosome, derived by recombination from the mother's two X chromosomes. The sons can, in this scenario, end up with anything from a complete half-identical X match to no X match at all, with any or all of the daughters, due to recombination and the random assortment of chromosomes.

    It is not the case that inheritance of the X chromosome "is exactly the same" as the inheritance of the autosomes! The general mechanism (meiosis) is the same, but in males, there is only one X chromosome to work with, and it is only passed on to daughters.


    • #3
      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!