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Cousin Match on X

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  • Cousin Match on X

    I am new to using the X chromosome and need some help.

    Will Female first cousins who share the same grandmother, be an identical match on the X Chromosome?

    If the brothers are half-siblings, would this still be the case as long as the share the same mother?

    I think the answer is Yes, the X should be identical between them since they share the same grandmother.

    Thanks

  • #2
    You haven't stated whether she is a maternal or paternal grandmother, but it doesn't affect things all that much.

    If their fathers are brothers there are three possibilities. Each of the fathers may have inherited an identical X-chromosome from their mother which was passed down unchanged from one of her parents. Another possibility is that they do not match at all - one received his maternal grandmother's X-chromosome and the other received his paternal grandmother's chromosome. The third possibility is that recombination occured and they have some matching segments but do not match the entire length of the X-chromosome.

    Whatever the makeup of their X-chromosome their daughters will receive this entire X-chromosome that their father has, which may or may not match.

    If they are related through their mothers then the mothers would have received identical X-chromosomes from their father, but may or may not have received the same X from their mother (same as the paragraph above).

    These mothers may then pass down a complete unchanged X-chromosome from either parent, or a combination of both.

    So the answer is no. Female first cousins with the same grandmother will not always be fully identical along the X-chromosome. They may be, but it wouldn't be all that likely. It's also possible they don't match at all.

    Comment


    • #3
      Follow up on Cousin X question

      Thanks a bunch for replying. I did a poor job of asking my question and explaining things. This would be the paternal grandmother.

      After doing a bit more reading, I found this snippet of information regarding two female cousins whose fathers are siblings with the same mother. Maybe I'm still missing something, but would this mean that the cousins would share at least 50% of X?

      Each female cousin's paternal X-chromosome is a 100% match to their shared paternal grandmother. This is because there is zero recombination between the X and Y chromosomes of the father. The father simply passes a non-recombined X-chromosome to his daughter. That non-recombined X-chromosome has the entire 196cMs of X-DNA which is passed down intact from the cousin's paternal grandmother.

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      • #4
        I don't know where you got the red quote, but it seems to be worded with the assumption that the two cousins in question, sharing the same paternal grandmother, also have the same father!

        Assuming the grandmother is Jane, I believe the actual scenario is that she has two sons, George 1 and George2. While the two Georges both have 1 X chromosome, which they must pass intact to any daughters they may have, the X chromosomes of George 1 and George 2 will only very rarely be identical. Their respective X chromosomes are the result of two separate and independent meiotic events, and the most likely outcome is that each one will get a recombinant X chromosome that is a different random combination of parts of both of Jane's X chromosomes.

        So, I would put aside the red text, permanently.

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        • #5
          I get what the red text is saying.

          If you looked at the chromosome browser George 1 and George 2 would both match their mother along the entire length of the X-chromosome (but do not necessarily match each other).

          George 1's daughter inherits this exact X-chromosome, so the chromosome browser would show that she matches her paternal grandmother along the entire length of the X-chromosome.

          Same applies to George 2's daughter.

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          • #6
            Thank you for finding the right way to parse that quote!

            What it says would be useful if the grandmother were available for testing: all her grandchildren through her sons would be a complete X match with her, even if they were only partial X matches with each other (or occasionally, not matching each other on the X at all).

            However, if the grandmother is not available for testing, not particularly helpful!

            Comment


            • #7
              Originally posted by John McCoy View Post
              Thank you for finding the right way to parse that quote!

              What it says would be useful if the grandmother were available for testing: all her grandchildren through her sons would be a complete X match with her, even if they were only partial X matches with each other (or occasionally, not matching each other on the X at all).

              However, if the grandmother is not available for testing, not particularly helpful!
              All her female grandchildren only (through sons) as males do not receive fathers X,
              both male and female grandchildren through daughters will be only partial matches to her on X (they will have a mixture of Grandfather also), although it is possible for them to receive their mothers full maternal X also which did not recombine with mother paternal X.

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