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Thread: New Ground for Me: The X Chromosome

  1. #21
    Registered Users
    Pittsburgh, PA, USA
    (U.S.) American

    United Kingdom Germany Bayern Catalonia France Ireland Switzerland
    To illustrate the idea that the source of an X chromosome segment can appear closer than is actually the case, I worked out X paths leading to a common ancestor of both my "3rd cousin" and me. (It turns out that we're related in a couple of ways, but the closest seems to actually be 4th cousins once removed.)

    For me, the path is

    Rachael Jones (1)> John "Jailor" Sizemore (2)> Mary Ann "Polly" Sizemore (3)> Hannah Louise Muncy (4)> my grandfather (5)> my mother (6)> me

    This represents six transmissions of the X chromosome, but only four times where two X chromosomes could recombine. In the two father-to-daughter transmissions, recombination wasn't possible. So while Rachael is my 4th great grandmother, as far as the X-chromosome is concerned she might as well be my 2nd great grandmother.

    For my cousin, the path is

    Rachael Jones (1)> William D. "Willis" Sizemore (2)> Rachel Sizemore (3)> Felix Sizemore (4)> my cousin's mother (5)> my cousin

    This represents five times that the X chromosome was transmitted, but only three times in which recombination could take place. So Rachael is my cousin's 3rd great grandmother, for for the X she is effectively his great grandmother.

    I definitely have to say that knowing the possible paths of the X chromosome helped a lot in this case; but you do have to have something to go on. Unfortunately, DNA can't entirely uncovering some sort of paper trail. But on the upside, it can allow you to see where the paper trail may not be making sense -- although that's true of autosomal DNA, and not just for the sex chromosomes. (It's just that the X chromosome can greatly reduces the number of possible paths.)
    The short explanation of my ancestry is British-German-Catalan, but it actually includes smaller amounts of French, Irish, Swiss, Choctaw and probably Cherokee. My avatar picture is of my father, his father, and his father's father. The baby in the picture is my eldest brother.


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