Tinsel Wing

Tuesday, July 18, 2006

Calico Women and Larry Summers: part deux

Okay, so assuming I haven't been tarred, feathered, and defenestrated: Where do calicos come into it?

This is the fun part. Calicos explain why males enjoy a standard of deviation that makes them more deviant and less standard than their opposite numbers.

As you probably already know, all calico cats are female. No toms ever sport that colorful patchwork fur. What you may not know is that all female mammals, including all women, are calicos. Here's how it works.

Most chromosomes come in two copies. The copy from Dad will differ in minor ways from Mom's copy, but they code for pretty much the same genes. A certain amount of that gene's product, not too much and not too little, is needed for the proper functioning of the cell. So all of the cell's deliriously complicated little gene expression mechanisms work together to guarantee that each chromosome pumps out enough messenger RNA to get half of the optimal amount of the protein in question built. Jointly, they fill the quota.

But when it comes to the sex chromosomes, there's a snag.

The Y's all right. Nobody, in the ordinary course of things, has more than one copy. Anything it produces, it will have to produce all of, so the gene activation machinery just doubles the order. But what to do with the X?

Sometimes (in girls) there'll be two copies of the X, sometimes (in boys) there'll be only one. But most of the X genes (unlike Y) have nothing to do with sexual characteristics. A protein that functions well at one concentration will starve or overwhelm its target process at half or double that concentration. It wouldn't do to have all the guys spending their lives in hypoglycemic shock, or all the girls with blood sugar counts off the charts. (Just a for instance, the gene for insulin lies elsewhere.)

Evolution's solution to the problem was to set the gene expression thermostat at male levels. And then, in the females, to turn off one X chromosome in every cell, so that its genes never get expressed. As it happens, it doesn't get around to doing this until the fertilized egg cell and its daughters have divided quite a few times. And then it picks at random which X chromosome to turn off. Half the cells will now express Dad's X genes, half will express Mom's. The embryo has become what's called a "chimera": its body is composed of two kinds of cells, which are genetically different. Two intermingled and interspersed parts of its body have, in effect, become fraternal twins.

The choices of X are frozen. As the cells divide and the embryo grows, each Daddy-X cell produces only Daddy-X daughters, and each Mommy-X cell only Mommy-X daughters. The fetus, and eventually the infant, is patched together from big clumps of cells of one kind or the other. How big are those clumps? Look at the coat of a calico cat, and you'll see the patches written out on her fur.

In humans, skin and hair color aren't coded on the X chromosome, sparing girls some serious wardrobe compatibility headaches. What has emerged from the human genome project, though, is the fact that X contains a higher than usual density of genes which are expressed only in neural tissue. Brain genes. Consequently, while a man's brain runs on only one parent's X-genes, a woman's contains regions expressing her mother's X-genes, and other regions expressing her father's. You could say that women have twice the brains.

One would expect this to have a smoothing effect. Any brain protein from the X that makes her brother particularly smart, or dumb, or impulsive, or cautious, would have its effects moderated in her case by the version of the same protein she inherited from her other parent. Presto! The lower standard of deviation, in IQ and what have you, among females.

Last I heard, they'd pinned down which X proteins are expressed only in nerves. Learning the functions of them all is going to be a long hard piece of gumshoeing. So my previous graf's final sentence is just speculation at this point. The rest is solid.

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