Yesterday I mentioned that we have invisible stripes and more information was requested. So without further delay…
Today’s Moment of Science… Stripey women and calico cats.
One of the earliest discoveries in the field of genetics was the concept of ‘sex linked’ traits. In the Fly Room at Columbia University, they bred and inbred fruit flies to bang out anything they possibly could from the wee bugs’ genetics. A couple of years of sitting around watching flies fuck eventually produced a generation of male fruit flies with mutant white peepers.
Sex linked traits might be more accurately described as X or Y chromosome linked traits. The genes controlling these characteristics are on the chromosomes that also determine sex; they have nothing to do with perceived masculinity or femininity. The X chromosome being the larger sex chromosome, there are more sex-linked traits that can be passed on via the X chromosome.
When babies start incubating in their primordial fuckpuddles, they gotta go from two to one to a whole fucking lot of cells. In an XX embryo that’s just a few days old, one X chromosome gets a pillow held over its face and is told “go quietly” in a process called gene silencing or inactivation. This process happens when you’ve got anywhere from a hundred to a thousand cells, leaving a mix of X chromosomes expressed cell by cell. It’s random as mallard which X chromosome makes the cut from one cell to the next.
As cells divide and the body grows, the same active X chromosome will continue to be expressed in new cells. A mosaic of each develops throughout the body.
“But Mrs. Auntie SciBabe,” I hear you think, “I am not a mosaic of anything except anxiety and ADHD.”
Ditto, but you see, you’ve already earned more stripes than that. Literally.
I told you that story to tell you this story.
So, Blaschko lines.
In human development, skin growth has been shown to follow a certain pattern. Dr. Alfred Blaschko compiled observations of 140 patients for a study he presented in 1901. As a dermatologist, he’d seen that rashes, birthmarks, and other skin issues on patients followed similar patterns in the skin. It didn’t follow any known bodily system like the nerves or blood vessels, it was just this pattern.
Dr. Blaschko did suggest the pattern could have “an embryonic origin,” but never got much past there. Given that it was 1901, not too shabby. We have more detailed mapping for this now, but it’s kept the name Blaschko Lines (or Blaschko’s Lines).
We all have this skin pattern, but on people with two X chromosomes? Because there’s been a mix of active X chromosomes growing along this pattern since the first week of growth, we get motherfucking stripes.
You can see them under UV light, or you can go to your local animal shelter. Because the fur patterns of calico and tortie cats are caused by the same phenomenon. Which is why almost all calicos and torties are female.
This has been your Moment of Science, signing off to snuggle my tortie, Lexi, in all of her stripey goodness.
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