David Bainbridge - The X in Sex: How the X Chromosome Controls Our Lives

THE X IN SEX:

HOW THE X CHROMOSOME CONTROLS OUR LIVES

DAVID BAINBRIDGE

HARVARD UNIVERSITY PRESS

CAMBRIDGE, MASSACHUSETTS

LONDON, ENGLAND

For Edward

Thanks for taking the Y off my hands.

I wasnt really using it much anyway.

And if you have any complaints about the X, blame your mother.

CONTENTS

PROLOGUE

1. MAKING A DIFFERENCE

INTERLUDE: WHAT IS IT, EXACTLY?

2. THE DUKE OF KENTS TESTICLES

INTERLUDE: HOW SEXY IS X?

3. THE DOUBLE LIFE OF WOMEN

EPILOGUE: THE CHOSEN ONE

FURTHER READING

GLOSSARY

INDEX

A woman is, as it were, a mutilated man, for it is through a certain incapacity that the female is female.

Aristotle, Generation of Animals

Thus man... is the active principle while woman is the passive principle because she remains undeveloped in her unity.

Hegel, Philosophy of Nature

One is not born, but rather becomes a woman.

de Beauvoir, The Second Sex

PROLOGUE

It has been six weeks now. Six weeks of tireless, frenzied activity since that sperm jostled its way into that egg. So little time spent in this warm, dark, womby home, and so much achieved.

First came those days of dividingceaseless splitting and splitting that turned a single fertilized egg into hundreds of tiny cells. Most of those cells went into making a protective hollow ball, but safe inside that ball lay another clump of cells with a very special destiny. That was four weeks ago, but how things have changed. The little clump of cells now looks recognizably like a tiny baby. It has grown, of course, but even more dramatic is the way it has organized itself into a child, with head, body, arms, legs, eyes, mouth. Really, all these bits and pieces have to do in the remaining thirty-four weeks before birth, is grow.

Yet there is one part of the baby that is still far from finished, and in many ways it is the most important. After all, the reason why nature makes us have babies is so that those babies can have their own babies. But even though so much of the babys body has been mapped out, the parts that will make future grandchildren are far from complete. Deep inside the embryos belly, just to the side of its kidneys, lie its gonadsthe organs that will eventually turn into ovaries or testicles and drive the childs sexuality up to birth, to puberty, and long into adulthood. The gonads are already crammed full of germ cellsfuture sperm or eggsbut at six weeks, they still do not know which way to turn. The embryo does not yet know if it is a boy or a girl. The jargon name for the gonads at this stage is indifferent, and somehow it describes them so well. They are, in fact, supremely indifferent. They are neither male nor female, but hovering in sexual limbo somewhere in between. All the rest of the sexual organs are equally undecided; simple tubular structures awaiting their cue to turn into male or female genitals.

So what is the spark of sexuality that makes a child a boy or a girl?

In this book, I hope I can show you how this spark drives us to become men or womenpeople apparently so different, but made from the same stuff. Although many people are aware of the principles by which we are allocated our sex, I suspect that few realize that this is a story rich in history, evolution, and philosophy that challenges our views of society. We humans use an unusual method to decide our gender, and it can have dramatic effects on the way we live our lives. It may help many of us become normal men and women, but it also consigns many to a life of disease, disrupts the everyday running of our body, and even forces women to live a bizarrely double life. The actual physical entity that causes all this upheaval is a little nugget of life called the X chromosome, and this is its story.

MAKING A DIFFERENCE

What did it all mean? Hermann Henking peered intently down the microscope and desperately tried to explain the strange behavior of the little purple flecks on the glass slide. It was 1890, and all over the world scientists were studying these flecks, yet still they seemed as mysterious as ever. Many believed that they held the key to the holy grail of biologythe mechanism that allows offspring to inherit characteristics from their parentsbut no one could prove it. As Henking sat in his dingy laboratory in Leipzig, he was about to make a discovery that was to revolutionize the science of inheritance, but he was also to set that science back ten years. His observations would lead directly to our modern understanding of sexuality and inheritance, but no one would realize the importance of his work for another decade. Indeed, Henking himself was to wait a further thirty years before another zoologist explained to him the magnitude of what he had achieved.

The 1890s were, in retrospect, a deeply frustrating decade. Charles Darwin had already shown, in his book On The Origin of Species, that external forces could make animal species change over time, so long as parents could bequeath characteristics to their offspring. Yet no one could say how all this bequeathing was done. Everyone knew that children often resembled their parents, and that the same seemed to be true of calves, foals, puppies, kittensin fact, if you hunted hard enough, family resemblance seemed to occur in any type of animal you cared to look at.

So inheritance seemed to be a ubiquitous phenomenon, and yet nobody could explain it. Many zoologists argued about which bits of sperm and eggs carried the instructions to make each generation resemble the last. Most agreed that there must be some physical means by which these instructions were imparted to a new individual, but a few still argued that inheritance was a spiritual, and not a physical processto believe that inheritance was somehow mundane might remove God from his place at the center of human affairs. And here was Henking, making a discovery that was to resolve this whole debate. He was finding the inheritance machine, and he did not even realize it.

Although it may sound rather unpromising, Henkings strained eye was fixed on chromosomes from the testicle of an insect called Pyrrhocoris. Twenty-five years earlier, a new microscopic dye had brought the chromosomes (colored bodies) into view for a generation of scientists hunting the inheritance machine. These chromosomes had been found to behave in an enticingly unusual way. Every cell in the body holds a set of chromosomes, and most of the time these chromosomes do not, to be honest, look very special. The time when chromosomes spring to life is when a cell starts to divide into two daughter cells. The chromosomes magically change from rather vague, diffuse things into a neat set of clearly defined little threads carefully arranged in the middle of the cell. Even more intriguingly, the chromosomes are then teased apart into two apparently identical bundles, one of which is then packaged into each of the daughter cells. It seemed that chromosomes must be doing something pretty important if they were so neatly partitioned between cells, so could they be the inheritance instructions that scientists sought?

Henking was not studying fly testicles because he was interested in fly testicles. Scientists can be strange people, but not usually that strange. One of the most intriguing features of chromosomes was apparent only in testicles and ovaries, where cells divide to make sperm or egg cellsthe cells that make the next generation. The cell divisions that made sperm and eggs seemed very different from the divisions that made all other cells. Instead of the single-step cell division that takes place when other cells are made, the chromosomes go through a strictly choreographed sequence of two successive divisions, and during these divisions, they are in a state of constant interaction with each other. Why should there be such an especially elaborate chromosomal dance when eggs and sperm are made, if chromosomes are not involved in inheritance?