If on the other hand sperm find their way into the

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Unformatted text preview: hen we tell the story of how a human baby came to be, we usually begin at the moment when sperm meets egg. Maybe that’s because we generally don’t think of eggs and sperm as human beings. Eggs carry most of what it takes to make a human being. In fact, eggs contain everything needed to make another human being except for twentythree chromosomes. Those twenty-three chromosomes come from a sperm, and that is pretty much all that the sperm brings to this blessed event. A human egg begins its life in an ovary as the end result of a mysterious process called meiosis. Meiosis is a peculiar type of cell division that eventually produces cells with twenty-three chromosomes. Twentythree is one-half the number of chromosomes found in all the rest of the cells inside of a human being. So we call sperm and ova haploid cells—half cells—and it takes two cells (egg and sperm) to create a complete human being whose cells each have forty-six chromosomes (diploid cells). About once a month in a sexually mature woman, one of the haploid eggs matures, slips into the Fallopian tube, and heads off toward the uterus—a relatively short journey, distance-wise, but a journey Where Our Sexes Come From 49 pregnant with possibilities. If the egg never encounters a sperm, then this ovulation leads to menstruation as the uterus sloughs the bed it had prepared for an embryo. If, on the other hand, sperm find their way into the vagina during coitus at the right time, the events that follow may be entirely different. Sperm are fertilization machines—twenty-three chromosomes and an eggbeater for a tail. Sperm exist for a single purpose—finding and fertilizing eggs; it is their raison d’etre. From the vagina, sperm head for the uterus, and from the uterus to the Fallopian tubes. Inside these moist tunnels, fertilization becomes possible. The first sperm to reach its goal augers into the egg and slams the door behind it, usually guaranteeing that only a single sperm will fertilize a single egg. Then things get really intense. The chromosomes carried by egg and sperm gather together, and a single diploid cell appears, a single cell from which all others will arise—the bones and tendons of the foot, the neurons of the amygdala, the lens of the eye, the crease of the lips, the fertile ponds of the genitals, the curious curl of the iris, the whorl of a fingerprint. Imagine the complexity of that process, which will produce hundreds of billions of cells, no two exactly alike, cells capable of all the things that human beings have ever done or thought, cells that can reach out or withhold, cells to listen with, cells to see with, cells to touch one another at the instant we most need to be touched, cells each more intricate than the most complex human creation. Imagine the possibilities for mistakes. During fetal development the opportunities for error, and the consequence for those errors, are as great as they will ever be. The leading cause of infant mortality is still developmental abnormalities—mistakes made along the way. And that way is so full of twists and turns and blind alleys that we know more about the surface of the moon than we truly understand about what drives and directs embryonic development. Yet that process affects nearly everything about us—our eyes, our ears, our feelings, our sanity, our toes, our bottoms, and our tops. And our sexes. In fact, in biologists’ minds, maybe in most of our minds, the process of embryonic development directly, inevitably, solely, and finally 50 Between XX and XY determines our sex. In fact, the one aspect of human development about which most people would agree that nurture plays no role whatsoever is human sex. Sex—boy versus girl—is all nature, we believe. In perhaps no other aspect of human development would we so quickly conclude that genes and chromosomes alone are solely responsible for a human trait. Even with things like eye color and hair color we anticipate environmental effects and changes during a person’s lifetime. If you have two X chromosomes, you are first a girl and then a woman. If you have an X chromosome and a Y chromosome, you are first a boy and then a man—period. Why we even believe such a tale about how sex is created is a mystery. We don’t think that way about our personal habits, muscles, height, or even sanity. We reserve very special, elaborate, carefully crafted closets inside our minds for thoughts about sex. And inside of those closets we have rules, rules that no one can break without consequence. For that reason it is worth recapitulating this story we most like to tell about how we become boys or girls—forgetting for the moment, as we so often do, those who never do become boys or girls. Before the newly formed embryo even leaves the Fallopian tubes, the cells that will become the gonads have separated from the others. Through the first four days and five rounds of cell division—until roughly the thirty-two-cell stage—the embryo grows as a solid ball of cells, a clump o...
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This document was uploaded on 02/04/2014.

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