Let's talk about sex! No, not
that kind of sex. I'll get to that eventually, but here the topic is the differentiation of the human species into male and female. A biology lesson.
We've all heard about chromosomes before, but here's a quick review. A normal
H. sapiens has 46 chromosomes in each cell. Of these 46, 44 come in 22 pairs of autosomes -- each chromosome has a partner that carries information about the exact same traits -- so that if, for instance, one of your chromosomes has a faulty gene for hemoglobin but the other is normal, then you will be a carrier for sickle cell anemia.
The sex chromosomes are a special case, though. They do not match, but code for completely different things. Women's two copies of the X chromosome come in handy in coping with the mutations that cause things like color blindness or hemophilia, but men get by just fine with only one. Men, on the other hand, have an entire chromosome of genes that women lack entirely. Interestingly, that means that if you just look directly at the DNA itself and don't take into account how it's expressed, a person has more in common genetically with a chimpanzee of the same sex than a human of the other (or so I've heard, but that may be an urban legend).
So here's a question: where exactly are the genes located that code for masculine physical and psychological characteristics, and where are those that tell your body how to develop in a feminine way?
It's easy to leave high school (perhaps even college, for that matter) with the idea that the way the Y chromosome makes a person develop into a man is pretty straightforward; it contains the genes for every masculine trait. But actually, it's not that simple. The Y chromosome is absolutely miniscule compared to the others and contains mostly just some information necessary for complete development of the testes. That, and one very important gene called SRY. SRY triggers the development of undifferentiated gonads into testes in a seven week old, human fetus. The testes then produce various chemicals, which control the rest of the process of sex differentiation (including maturation at puberty). In the absence of a functional SRY gene, the gonads will turn into ovaries, which produce the hormones that control development into a female.
The actual development of the sex organs is complicated. For that, I point you to a wonderful
interactive website which describes it far better than I could, with diagrams and everything.
That's what the Y does, but what about the X? Well, the interesting thing about X is that it's actually expressed equally in males and females, despite females having twice as many copies. The way that's done is by deactivating one copy in each cell. The deactivated copy is known as a Barr body, and which one it is will vary at random between areas of your body. This phenomenon is what causes some female cats to have calico or tortoiseshell coat patterns -- they inherit one allele for black and one for orange, and whichever one is expressed in a given area will give it that coat color. Males are the same color all over (or white and that color, but the white comes from a completely separate gene) because they only get one or the other on their single X chromosome.
So, that's how sex works . . . in humans and most mammals, that is. The rest of the animal kingdom has got other creative ways to decide which sex an embryo should become. Birds use sex chromosomes known as W and Z, where
males are the ones with two of the same and
females inherit the sex-determining W chromosome. Fish and reptiles are even more interesting, with some reptiles differentiating by temperature, and some fish having the ability to change sex based on their social status:
clownfish live in groups with one dominant female, one breeding male, and several lower-status males who remain prepubescent while in that position. If the female dies, the dominant male changes into a female and takes her place. Makes you look at
Finding Nemo slightly differently, doesn't it? And then insects have everything from hermaphroditism to
haplodiploidy. Drosophila (fruit flies) use X and Y but differentiate solely based on the number of X chromosomes they have.
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