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    February 4, 2015

    When a female embryo is four days old it consists of just 100 cells. At this point the x-chromosome from Mom and the one from Dad are both active. But in order for proper development to occur, one of the x chromosomes must be switched off.

    Through a tiny molecular battle within each cell, one of the x-chromosomes wins and remains active while the loser is deactivated.

    This is done by wrapping the DNA tighter around proteins, modifying histone tails, and DNA methylation – molecular markers to indicate this DNA should not be read.

    What’s surprising is that it’s pretty random which x chromosome wins – sometimes it’s Mom’s and sometimes it’s Dad’s. So when a female is just 100 cells big, her cells have a mix of active x-chromosomes, some from Mom and some from Dad.


    Epigenetics means women have different active x-chromosomes in different cells.
    Animation courtesy of
    Music by Amarante:
    Animation: Etsuko Uno
    Art and Technical Direction: Drew Berry
    Sound Design: Francois Tetaz & Emma Bortignon
    Scientific Consultation: Marnie Blewitt
    Courtesy of Walter and Eliza Hall Institute of Medical Research:

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