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X Matches all coming from woman's mother?

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  • X Matches all coming from woman's mother?

    FTDNA shows my wife's matches on her X chromosome being a 100% match to her mother's sister's X chromosome. Is FTDNA really trying to say that she got all of her X DNA from her mother and nothing from her father. When I say 100% match, I am saying that FTDNA shows that match with her aunt from end-to-end of the X chromosome. Is this real science or just ???

  • #2
    No, FTDNA is not saying what you propose. Females have two X chromosomes, one from their father and one from their mother.
    Yes, it is real science, but this may be a relatively rare situation, or one that is hard to see when only one relative is used for comparison.

    The Chromosome Browser does not show two of each chromosome, because there is no way to determine which segments came from the match's maternal or paternal chromosome. It shows only one representative bar for each of the chromosomes, 1-22 plus the X chromosome. How the matching segments display depends upon how the matching person is related. If you were able to compare your wife to her father, she would definitely also show a full chromosome match on the X as well as all the other chromosomes, because a parent gives each child a complete set of full autosomal chromosomes (except a father does not pass an X to his son). But if your wife compared how she matches to a brother or sister, or a different maternal aunt or uncle, it might only show partial matching on the X, or any of the other chromosomes. This is because those relatives may not have received a non-recombined X from their mother, but a recombined one, so will only match your wife partially on the X.

    Almost always, the one X chromosome that a mother passes on to a child is a recombination of her paternal and maternal X chromosomes. In some cases, however, she can pass down one of her X chromosomes without recombination. It is possible that both your wife's maternal grandmother passed down a non-recombined X chromosome to her two daughters (your wife's mother, and the mother's sister), and your wife in turn received this same non-recombined X from her mother. That situation would be rare; someone else may post information about just how rare it is. But, it would be one explanation for why she would match her aunt on the entire X chromosome. All of these women have another X chromosome, from their fathers. I believe that there are situations where it appears that one X has been passed down without recombining, but if comparisons are done to and between other maternal relatives who have done autosomal testing, you would see that it is not the case.

    You might find this webpage by Jared Smith helpful, "X Chromosome Recombination's Impact on DNA Genealogy." It has two relevant sections: "Understanding Recombination and Non-recombination," and "Non-recombination and Family Lines." The latter section illustrates how a full X chromosome can get passed down from mother to child.
    Last edited by KATM; 4 October 2018, 08:59 AM.


    • #3
      Full sisters will share their entire X from father as well as match partially on their maternal X from mother (each has a random recombination of mothers two X's).
      When your mother passed on a random recombination of her two X's to you, you would have received part of her paternal X (section will match with your aunt), and in regards to her maternal X you inherited, you most likely inherit a section that she also happen to share with your Aunt. Thus it may "look" like you are sharing a complete single X with your aunt, but in reality you are matching her on part of her paternal X as well as matching her on part of her maternal X.
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      Last edited by prairielad; 4 October 2018, 09:59 AM.


      • #4
        Thank you so much --- The explanation makes sense of the FTDNA presentation.