Epigenetics means women have different active x-chromosomes in different cells. Animation courtesy of http://wehi.tv
Music by Amarante: http://bit.ly/VeAmarante
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: http://wehi.tv
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.