kengr

Specifically, I've been doing some rough arithmetic.

So, the premise of mpreg is that people who have penises and testicles and XY chromosomes can get pregnant, sometimes or all the time. Weirdly, I've never encountered... uh... ffpreg, in which people with ovaries and vaginas and XX chromosomes can get other people pregnant. But anyway, let's assume it exists too. Which raises an interesting question about heterosexual couples - can they both get pregnant and also impregnate each other? Should we just assume that they're all hermaphroditic and that "male" and "female" are simply gender markers with no reference to physical sex?

Well, if we dig into that, we'll be here all night.

Anyway, in a high school biology textbook, virtually all people are either XX or XY. In real life it's not quite like that, but it's close enough for my math, and I don't want to make my math harder anyway. This math is going to be a lot easier if I ignore things like "reality" and "gender", so. Physical sex is one of the features of human existence that shows simple Mendelian inheritance patterns - it's not a multi-gene trait, I mean.

Now, back before conception, the gametes (sperm and egg) each contain half the genes of the parent, which are potentially half the genes of the new baby. If there is a baby. The egg cell will have an X chromosome, because the parent who contributes an egg only has X chromosomes, and the sperm will have either an X or a Y. In real life, there may be a screwup and you can get an embryo and thence a child with a weird combo like "X0" or "XXY" or whatever but, importantly, you can't have life without at least one X chromosome. It doesn't work.

And if we were to write a punnet square for sex, we'd get four options: X_mom1X_dad, X_mom2Y, X_mom1X_dad, X_mom2Y - half girl, half boy. If there are 600 babies born to such couples every year, we expect 500 xx and 500 xy. (Actually we don't expect that. If there is no artificial intervention then we get slightly more boys than girls, I don't know why. If there *is* artificial intervention then we tend to get lots more boys than girls, and everybody knows why. But this is not just high school bio, it's also high school math, or maybe middle school math.)

But let's look at a punnet square for a hypothetical mm couple, and this time without subscript because that's a hassle - XX, XY, YX, YY. One of those things does not work! So, taking away all the YY conceptions, they cannot survive, our options are XX, XY, and YX. If it doesn't matter who contributes the Y chromosome and who contributes the X chromosome then mm couples have a 2/3 chance of producing boys, which is to say, if there are 600 babies born to such couples every single year then we expect 400 boys to 200 girls, which is a just insane ratio.

However, if we also allow ffpreg... well, all their babies are going to be XX, right? So if mm couples and ff couples exist in the same ratio, and if they also reproduce the same amount, then out of our 1200 yearly babies with same-sex parents we'll see 800 girls and 400 boys.

This really shouldn't be enough to screw up the sex ratio of the entire population if the proportion of same-sex couples is about the same as it is in reality land. Same-sex couples are literally queer. There's just not as many people interested in it! But the people who write mpreg have a strong tendency to write a lot more same-sex couples than het couples. So maybe in the mpreg-verse, nearly everybody is in a same-sex relationship and hardly anybody is in an opposite-sex relationship. In this idyll, women outnumber men by a huge percentage. I hadn't meant to conclude by linking to The Matter of Seggri, but why not.

I shouldn't have to switch from a/c to heater and back again!

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