If your interest in XXY is only that the second X is insignificant and that you are really an XY Male because you administer exogenous testosterone and by association, it makes you look more masculine, then there’s every possibility this book will not be of interest to you. If on the other hand, you have a keen interest in seeking answers as to what impact the second X might have on your wellbeing or what it is that makes us who we are, then you will most likely find this fascinating. 

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 dividing—ceaseless splitting and splitting that turned a single fertilised 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 recognisably like a tiny baby. It has grown, of course, but even more dramatic is the way it has organised 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 baby’s body has been mapped out, the parts that will make future grandchildren are far from complete. Deep inside the embryo’s belly, just to the side of its kidneys, lie its gonads—the organs that will eventually turn into ovaries or testicles and drive the child’s sexuality up to birth, to puberty, and long into adulthood. The gonads are already crammed full of germ cells—future sperm or eggs—but 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 women—people 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.