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Female 1st cousins

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  • Female 1st cousins

    Should two female first cousins match on the X chromosome?

    My first cousin and I (both females) descend from the same grandparents. My father and her mother are siblings. Shouldn't my 1st cousin and I match on the X chromosome by having received some X from our common grandmother?

  • #2
    Not always, it just depends what each of your parents received and then passed on to you guys

    You would have received your fathers X intact, which is from his mother only, random mixture of her two

    your cousin would have received a random mixture of her mothers two X (mother received one from your grandfather and random mixture of your grandmothers two)

    So depending on how much was shared between brother and sister on their maternal X, shared segment may not have been passed down to your cousin. She could have inherited your grandfathers X where her mother and your father shared.
    Last edited by prairielad; 16th February 2017, 12:44 AM.

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    • #3
      Originally posted by prairielad View Post
      Not always, it just depends what each of your parents received and then passed on to you guys

      You would have received your fathers X intact, which is from his mother only, random mixture of her two

      your cousin would have received a random mixture of her mothers two X (mother received one from your grandfather and random mixture of your grandmothers two)

      So depending on how much was shared between brother and sister on their maternal X, shared segment may not have been passed down to your cousin. She could have inherited your grandfathers X where her mother and your father shared.
      Thanks prairielad. I have a similar question which probably has the same answer but just want to make sure I understand this correctly.

      I have a half sister. We have the same mother but different fathers. We do not share on the X chromosome.

      I understand we would each receive an X from our respective fathers and each receive an X from our common mother.

      So, I guess her two X's could have come from her father's mother and hypothetically our mother's father (or mother's mother). And my two X's could come from my father's mother and our mother's mother (or mother's father).

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      • #4
        Originally posted by Tenn4ever View Post

        ..........

        So, I guess her two X's could have come from her father's mother and hypothetically our mother's father (or mother's mother). And my two X's could come from my father's mother and our mother's mother (or mother's father).
        Correct, each may have inherited mothers opposite X which did not undergo recombination.

        Or each have a crossover point in close proximity
        ie)you inherited mother paternal X at beginning of your maternal X, sister inherited mothers maternal X at the beginning of her maternal X.
        Each have a crossover point close together, where you switch to inheriting your mother maternal X and sister switches to inheriting your mothers paternal X.

        With out a 3 sibling reference, it will be one or the other

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        • #5
          Thanks. If the X is there on an autosomal match it's a nice PLUS to use with the X chart.

          However, if it doesn't show up it doesn't mean that the match isn't there on the mother's line. Just that finicky X didn't follow the course we hoped it would

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