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How Full Sibs Match X DNA

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  • How Full Sibs Match X DNA

    There seems to always be questions about how people share DNA on their X chromosome. Here are a few guidelines.

    Full Siblings.
    Sister to Sister will share 100% because they have the exact same copy of X from their father. It doesn't actually matter if they share their mother's X, since their father's X is 100% the same for both of them they will appear to share the full cM amount. Since the test is half identical matching, only the full 100% share will be counted and not the additional amount shared by their maternal X chromosome (females have XX, one paternal, one maternal).

    Sister to Brother, or Brother to Brother, will share anywhere from 0% - 100% of their X with their siblings. Sons do not receive X DNA from their father so this situation means only the full siblings' maternal X is compared, unlike sister to sister where both their paternal and maternal X are compared. This is because a mother passes down X chromosome DNA by recombination to her children. Recombination means they will not always receive the same mixture of their mother's XX being recombined to their one maternal X chromosome. Only when recombination places the same ancestral strands in the same locations per child will the sibling to sibling comparison show shared X DNA.

    Another way to look at this (sister to brother or brother to brother X sharing) is to view everything in terms of maternal grandparents. If a brother has X DNA from his maternal grandfather and his sister has X DNA from her maternal grandmother, the two siblings will not share X DNA in that location. If at a different place on their X chromosome they both have X DNA from the same maternal grandparent (either one), they will share that strand. This assumes the grandparents are not cousins to begin with of course.

    Going back to the sisters, since their father copied the same X to both daughters, they will also match 100% to his mother, their paternal grandmother. They would not match their paternal grandfather.

    More to follow a different day.

    Last edited by mkdexter; 27 November 2013, 12:42 AM.