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Thread: X Match Question

  1. #11
    Registered Users
    North Carolina

    Scotland Spain
    Thanks! Finally remembered my password here.

  2. #12
    Registered Users
    Pittsburgh, PA, USA
    (U.S.) American

    United Kingdom Germany Bayern Catalonia France Ireland Switzerland
    Ann is the expert here, but one reason full sisters might not have any complete matching on the X chromosome (matching on both X chromosomes in the same region) is that they might have inherited opposite X chromosomes from their mother.

    As others have said, typically the X chromosome a mother passes on to a child will be a recombination of both of her X chromosomes. Typically, but not always. It's possible for her to pass on a copy of either her maternal X, with no DNA from the opposite chromosome.

    This actually happened to two of my siblings -- one brother and one sister. They match across the full length of their maternal X chromosome, which happens to have come from our mother's father. (This has been confirmed by an abundance of matches to cousins on our mother's father's side, across the entire chromosome. In addition, I match these two siblings on all of the X except for about a 20 cM region surrounding the centromere, where I match cousins on our mother's mother's side.)

    Now, these two siblings happened to have received an X chromosome from the same grandparent, but it could as easily have happened that they received an X chromosome from opposite grandparents. In that case, despite being full siblings, they would have no matching on the X chromosome.

    Obviously, if this were two full sisters instead of a sister and a brother, they would still match each other on their paternal X chromosome. But, what this means is that you can't presume that the absence of any complete matching between sisters means they're half sisters; or on the other side, that two sisters with half matching across the entire X, must have inherited that X from a common father. As I've indicated, it is possible for siblings to inherit an identical X chromosome from their mother.

    EDIT: As an aside, you might think that with six children, we'd collectively have inherited all of both of our mother's X chromosomes. We nearly did, except that none of us managed to inherited the region beginning at 144139922 and ending at 155092794 (which 23andMe shows as the end of the chromosome) from our maternal grandmother.
    Last edited by geebee; 06-29-2017 at 04:22 AM.
    The short explanation of my ancestry is British-German-Catalan, but it actually includes smaller amounts of French, Irish, Swiss, Choctaw and probably Cherokee. My avatar picture is of my father, his father, and his father's father. The baby in the picture is my eldest brother.


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