Understanding the 'Female' Chromosome

Excerpted from
Woman: An Intimate Geography
By Natalie Angier

Keith and Adele fought all the time, like a pair of tomcats, like two drunken lumberjacks. Keith would find grist for the arguments in his reading. He read widely and thirstily, and sometimes he would come across a stray fact that fed his theorizing about the natural cosmology of male and female. Males are the seekers, he had decided, the strugglers and the creators; they build all that we see around us, the artifactual world of towering cities and invented divinity, yet they suffer for their brilliance and busyness. Females are the stabilizers, the salve for man's impatient expansionism, the mortar between bricks. Nothing surprising there: it's a familiar dialectic, between the doers and the be-ers, the seethers and the soothers, complexity and simplicity.

Then one day Keith read about chromosomes. He read that humans have twenty-three pairs of chromosomes and that the pairs of chromosomes are the same in men and in women, with the exception of pair number 23-the sex chromosomes. In that case, women have two X chromosomes and men have one X and one Y. Moreover, a woman's two X chromosomes look pretty much like all her other chromosomes. Chromosomes resemble Xs. Not when they're inside the cells of the body, at which point they're so squashed and snarled together they resemble nothing so much as a hair knot. But when they're taken out of the cell and combed apart for viewing under a microscope by a geneticist or a lab technician who is checking a fetus's chromosomes as part of amniocentesis, they look like fat and floppy Xs. So women have twenty-three pairs, or forty-six, of these X-shaped structures, while men have forty-five Xs and that one eccentric, the Y chromosome. The Y physically resembles the letter it was named for, being stubby and tripartite and quite distinct in shape from all the other chromosomes in the cell.

It struck Keith that even on a microscopic level, even as inscribed in the genetic clay from which human beings are constructed, men demonstrate their edge over women. Women have as their sex chromosomes two Xs: monotony. The story we've heard before. Men have an X and a Y: diversity. Genetic innovation and an escape from primal tedium. The Y as synecdoche for creativity-for genius. And so he said to Adele, The chromosomes prove the case for male superiority. You have two Xs and hence are dull, while I have an X and a Y and am accordingly interesting.

Neither Adele nor Keith knew much about genetics, but Adele knew enough to recognize mental manure when she smelled it. She dismissed his theory with a sneer. He grew angry at her refusal to submit to his logic. The argument escalated, as their arguments always did. Keith wasn't talking about all men, of course, but about himself. He was insisting that his needs and insights took precedence over Adele's, and that she acknowledge as much. She refused to surrender.

Of the many arguments that my parents had in the theater of our apartment before the reluctant audience of their children, this is the only one whose substance I remember. The clash of the century, Y versus X. I remember it in part because it seemed so oddly theoretical, and because it was the first time I heard an argument put forth for all-around, across-the-board male dominance. I took it personally. My feelings were hurt. It was one thing for my father to attack my mother-that I was accustomed to. But there he was, describing all females, including me, as chromosomal bores.

The chromosome case remains very much open, a source of irritation and debate. In some ways, sex is fundamentally determined by the sex chromosomes. If you're female, you're assumed to have a pair of Xs tucked into just about every cell of your body, along with a set of those twenty-two other pairs of chromosomes. If you're a male, you know of your Y and you just might be proud of it, as your molecular phallus, and for the koanic wordplay of it: Y? Why? Why? Y! The sex chromosomes tell a technician-and you the parent, if you choose to know-whether the fetus under scrutiny in an amniocentesis screen is a girl or a boy.

So in one sense the demarcation between X and Y is clear, clean, an inarguable separation between femaleness and maleness. And my father was right about the predictability and monochromaticity of the female chromosomal complement. Not only will you find two X chromosomes in every body cell of a woman, from the cells that line the fallopian tubes to the cells in the liver and brain, but break open an egg cell and look within the nucleus, and you'll find one X chromosome in each (again with the other twenty-two chromosomes). It is indeed the sperm cell that can add diversity to an embryo, and that determines the embryo's sex by delivering either another X, to create a female, or a Y chromosome, to make a male. X marks the egg. An egg never has a Y chromosome within it. An ejaculate of sperm is bisexual, offering a more or less equal number of female and male whip-tailed sperm, but eggs are inherently female. So in thinking again about the mirrors into infinity, the link between mother and daughter, the nesting of eggs within woman within eggs, we can go a step further and sec the continuity of the chromosomes. No maleness tints any part of us gals, no, not a molar drop or quantum.

But of course it is not that simple. We are not that simple, appealing though the idea of a molecularly untainted matriline may be. Let us consider the nature of the sex chromosomes, the X counterpoised against the Y. To begin with, the X is bigger, much, much bigger, both in sheer size and in density of information. The X chromosome is in fact one of the largest of the twenty-three chromosomes that humans cart around, and is about six times larger than the Y, which is among the tiniest of the lot (and it would be the smallest of all if it didn't have some nonfunctional stuffing added to it just to keep it stable). Gentlemen, I'm afraid it's true: size does make a difference.

In addition, many more genes are strung along the female chromosome than along its counterpart, and it is as a shoetree for genes that a chromosome takes on its meaning. Nobody knows exactly how many genes sit on either the X or the Y chromosome; nobody yet knows how many genes, in total, a human being has. Estimates range from 68,000 to 100,000. What is incontestable, though, is the vastly higher gene richness of the X than of the Y. The male chromosome is a depauperated little stump, home to perhaps two dozen, three dozen genes, and that's the range scientists come up with when they're feeling generous. On the X, we will find thousands of genes, anywhere from 3,500 to 6,000.

What does this mean to us women? Are we the mother load of genes, so to speak? After all, if we have two Xs, and each X holds about 5,000 genes, whereas a man has but one X with 5,000 genes and a Y with its 30 genes, then you don't even need a calculator to figure that we should have about 4,970 more genes than a man. So why on Gaia are men bodily bigger than we are? The answer is among the neat twists of genetics: all those extra genes are just sitting around doing nothing, and that's just the way we want them. In fact, if they were all doing something, we'd be dead. Here is what I love about a female's X chromosomes: they are unpredictable. They do surprising things. They do not act like any of the other chromosomes in the body. As we shall see, to the extent that chromosomes can be said to have manners, the X chromosomes behave with great courtesy.