Barr Bodies and XISTence

Dear CF,

Courtesy of my old biology textbook, a way in which we ladies are a special kind of parental battleground:

You have cells. In any one cell you’re expressing either your mom’s X chromosome or your dad’s. Not both. X-chromosome-wise, any cell on you is either all mom or all dad. You are a calico cat.* So, Mama X and Papa X have the same fight over and over in each one of your cells until one gives in and becomes inactive. This inactive X chromosome is called (I kid you not) a Barr body. 

It gets better: a Barr body has to be inactivated by attaching methyl groups to cytosine, one of the DNA groups. The gene that seems to make that all happen is called XIST, which stands for X-inactive specific transcript. And which, said aloud, is EXIST.



*Literally: calico cats have orange and black patches because one parent’s X-chromosome won in the black bits and the other’s won in the orange bits. This is also why there are no male calicos.


5 Responses to Barr Bodies and XISTence

  1. Carla Fran says:

    Do boys automatically only express one parent’s X chromosome? Is it always the same parent? Why the lotto/battle for ladies and staid filled-out scantron for the boys?

  2. Millicent says:

    Since boys only get one X chromosome, mom’s, there’s no battle. Which also means they’re more subject to mutations or diseases carried on the X chromosome. Like, why more men than women are colorblind, or have hemophilia. Women need two bad genes to express a recessive genes; with boys, all they have is just the one.

    • Carla Fran says:

      Ah. This might sound idiotic, but do their cells have Y chromosomes? Do the Ys fight it out? I think I better go back and read something basic on how people are made. Natalie Angiers does a good job of summing this up, if I remember…which is ashame that I can remember that her summary was good, just not what the summary itself was.

  3. Millicent says:

    Their cells do all have Y chromosomes, but just the one that comes from Dad and makes them a boy. So there’s no fight. Girls are XX, boys are XY. The Y chromosome is teeny-tiny compared the X chromosome and carries way fewer genes, but it, like the X, is the only copy they get, and those are unique copies of those genes. So boys are hybrids of Mom and Dad too, but there isn’t a fight for supremacy since the two sex chromosomes are complementary, not redundant.

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