Nature is analog, not digital

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A bit off topic, but it's been on my mind lately.

XX and XY are not the sex-absolutes you may think it is. They're the two most common bins, but they're far from the only genetic bins that humans end up in. Many, many people have been surprised when examining genes to determine "true" sex, often unhappily, and often complicatedly as a genetic condition a test wasn't designed to handle is encountered (how do you type XXY?).

What else is there out there?

Possibly the most famous is Androgen Insensitivity Syndrome (which comes in 'complete' and 'partial' varieties) in which a mutation on the hormone receptor for Testosterone either doesn't work or only partly works. Babies with C-AIS will end up with an F on their birth-certificate because that's what they look like, and they'll go through a normal female puberty even though they're still producing Testosterone.

That's because the liver does this neat trick called armoatization in which excess Testosterone is converted into Estrogen. This is why some perfectly normal teenage boys end up with gynecomastia, as all that surging Testosterone (puberty does that) causes a bit of it to convert.

Anyway, AIS girls develop in the womb along female patterns. The testes are still there, they're just not well developed. They also won't develop a uterus, since it wasn't there to begin with. Because of this, they won't menstruate but in every other way will look like any other girl (if a bit taller).

P-AIS is less definite, and is where some Intersex conditions come in to play.

I remember a scandal in the 90's when genetic testing for maleness was introduced among female Olympians, and they found two who tested male because of this. This was an extremely unpleasant surprise for them, as they'd both been competing at the world level for a while.

Next up is Klinefelter syndrome, which is an individual with an extra X chromosome to make XXY. And sometimes even more chromosomes get tacked on depending on what happened. These babies will most likely get an M on their birth-certificate, but development is where the differences begin to show. Testosterone production is reduced compared to XY males, but is still elevated compared to XX females.

In the same vein we have XXYY males. Those extra chromosomes aren't good things to have, but it does show up often enough we know about it.

The thing that breaks peoples brains is mosaicism, in which one person can have two different genomes. People with this can have a heart with one set, and an ovary with another, or eyes with different colors. One type of Turner Syndrome involves a mosaic of -X and XY (where -X is a missing X, they're short one). Depending on what tissue you take for typing, that individual may come up as either Turner-Female, or Male.

A slightly different version of this is chimerism, in which the two genomes came from two different zygotes. This can lead to fun things like true hermaphrodism if the reproductive parts of both individuals end up in the same body, and may have already allowed human parthogenesis. As with mosaics, these individuals can sex-type differently based on which tissue you take for testing.

If you ever wanted to see what a highly complex, failure accepting system looks like... biology. It's amazing we get anything done with all those transcription errors.

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