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You Are your Child’s Sex (?)

February 26, 2015

It’s been a while since Ultraphyte blogged on biological sex. Since Brain Plague in 2000, I’ve felt there was little more to be said on the postgender world. However, trust the cell biologists to reveal twists even more bizarre than science fiction.

For perspective: Back in the sixties, we were taught that people came in two sexes and four crayon colors (brown, red, yellow, white). Now we know that sexes, like colors, are a spectrum, like infrared through visible and UV. Some examples, in this open-access Nature review:

  • People are mosaic–perhaps 1% of us. Mosaic means we have large chunks of cells with a chromosome count different from other chunks of cells. So, maybe, your womb is female (XX) but your legs are male (XY). Or your testes are male (fathering children), then your surgeon “discovers” a womb tucked behind.

How can this happen? Several ways, each more bizarre than the last:

  •  Cell divisions in the early embryo make a mistake called “non-disjunction”; that is, at mitosis, chromosomes replicate but both copies go over to one daughter. So, for instance, YX –> YY XX –> daughter cells Y and YXX instead of YX, YX. The Y cell dies; but the YXX can recover by spitting out the Y, leaving XX. Now, a part of the body continues developing YX (the original cell line) whereas the others go XX.
  • A pair of fraternal twins (XY and XX) start out on their own, but then stick together and “merge” into one body. Now you genetically consist of  two different people, with two different chromosome sets.
  • Your autosomes (all the chromosomes other than X or Y) carry other sex-regulating genes–dozens of them. Any one of them can go missing at cell division, leaving you with some other kind of mosaic, say a male body that “ignores” the screaming male hormone. You end up a super beautiful female (outside) without functional reproductive organs.

It gets weirder. When you’re pregnant, what becomes of all those fetal cells that wind up in your own blood stream–enough for a blood test that precisely details your child’s sex and any genetic defects? Virtually 100% of mothers are mosaic with their children’s cells.

Some of those fetal cells wind up part of your tissue, even entering your brain and hooking up with your own neurons. So, decades later, you still have your child’s cells forming part of your brain. Your child, too, has some of the mother’s cells. So, a mother and a male child each share part of each other, including each other’s gender.

Where this all leads, we don’t yet know, but here’s a valentine for someone who knows why.


  1. March 1, 2015 10:39 am

    I’ve been attending some “progressive nightcaps” put on by my classmates and I love how during introductions we are asked to say our name and what gender pronouns we prefer. Life is so much more beautiful when we acknowledge all the little variations that piece together who we are.

    • March 1, 2015 4:29 pm

      Dan, I’m glad you’re continuing the kinds of exploration we do at Kenyon. The biomedical and social aspects of gender diversity are especially important for physicians to know about.


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