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    How large should the X match be for this relationship.

    Tom and Mary are half siblings who share a mother.
    Tom has a daughter.
    How much X DNA will be shared between Mary and Tom's daughter?

  • #2
    Originally posted by Cats View Post
    How large should the X match be for this relationship.

    Tom and Mary are half siblings who share a mother.
    Tom has a daughter.
    How much X DNA will be shared between Mary and Tom's daughter?
    I meant to post that according to the browser they share 1.4 cM and 675 snp's. Isn't that low?

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    • #3
      Tom has only one X chromosome, so his daughter should be a complete (half) match with him on the X chromosome, for the entire length of about 196 cM.

      In other words, both Tom and his daughter having the same X chromosome, should match Mary, Tom's maternal half sister, to exactly the same extent.

      As for Tom and Mary, they should each inherit one X chromosome from their shared mother, but because of the random assortment and recombination of the 2 X chromosomes of that shared mother, it is POSSIBLE for Tom and Mary to share any amount from 0 to 196 cM on the X chromosome. Those extremes should be rare, of course, but there have been many reports of siblings ending up either with exactly the same X from their mother, or else of ending up with completely different X's from their mother.

      Yes, 1.4 cM shared on the X chromosome for maternal half-siblings is low, but it is not impossible. Before concluding there is something amiss, it would be useful to examine the degree of autosomal matching (it should be consistent with half siblings) and whether there is a reasonable degree of matching with known maternal relatives.

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      • #4
        Tom is dead and his daughter is Nancy.

        Mary (tom's half sister) is Nancy's half aunt and they share 72 autosomal matches.

        This half brother information is a very new discovery. We assumed they were full siblings but the numbers for Nancy and Mary were not adding up. Therefore I located a woman in another country who should have been my mother's first cousin on her father's side and had her tested, she did not match my mother. Sadly this kit is one of ones that is stuck in processing so I do not have FtDNA's results yet. Thankfully, I was able to upload the raw data to Gedmatch and get the answer I was dreading.

        I plan on writing more about it in another thread but the X DNA does not seem to be acting as expected. Thus when this information first came to light many years ago, the people on this board conjectured that they shared fathers and had different mothers.
        Last edited by Cats; 28th March 2018, 03:21 PM.

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        • #5
          Mary and Nancy should share about 12.5% and 840 cM. There is a range above and below. So between 600 and 1200cM would definitely be consistent with half-niece. And it could be higher or lower but less likely.

          The X is less reliable for lots of reasons. For one thing, you are only looking at one chromosome whereas Autosomal looks at 22, so you get a better look. And the X is just plain quirky.

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          • #6
            If Tom and Mary are children of the same mother, each of them got one X chromosome from her. Usually a mother's 2 X chromosomes (one she received from her father and the other from her mother)get together and swap a few segments when egg cells arer formed, so that the child receives an X chromosome which has segments from both of the woman's parents. This is called recombination.

            Occasionally recombination fails to occur and the child gets one of the mother's X chromosomes entire, exactly as she inherited from either her father or her mother.

            I suppose it's very remotely possible that recombination did not occur when the egg cell that became Mary was formed and did not occur a again when the egg cell that became Tom was formed and one received the chromosome their mother got from her mother and the other got the one she received from her father. But I find it almost beyond belief that recombination would have failed to occur in both instances.

            Tom's daughter, of course, received the entire X chromosome he got from his mother. As he only had one X recombination could not occur.

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