a theory that does. He argues that because the father's sex chromosome determines the sex of the offspring — if he passes on an X
rather than a Y chromosome, the offspring is female — so paternal X chromosomes are found only in females. Therefore, behaviour that is characteristically required of females should be expressed only from paternal chromosomes. If they were also expressed from maternal X chromosomes, they might appear in males, or they might be overexpressed in females. It therefore makes sense that maternal behaviour should be paternally imprinted.16
The best vindication of this idea comes from an unusual natural experiment studied by David Skuse and his colleagues at the Institute of Child Health in London. Skuse located eighty women and girls aged between six and twenty-five who suffered from Turner's syndrome, a disorder caused by the absence of all or part of the X
chromosome. Men have only one X chromosome, and women keep one of their two X chromosomes switched off in all their cells, so Turner's syndrome should, in principle, make little difference to development. Indeed, Turner's girls are of normal intelligence and appearance. However, they often have trouble with 'social adjustment'. Skuse and his colleagues decided to compare two kinds of Turner's girls: those with the paternal X chromosome missing and those with the maternal X chromosome missing. The twenty-five girls missing the maternal chromosome were significantly better adjusted, with 'superior verbal and higher-order executive function skills, which mediate social interactions' than the fifty-five girls missing the paternal chromosome. Skuse and his colleagues determined this by setting the children standard tests for cognition, and giving the parents questionnaires to assess social adjustment. The questionnaire asked the parents if the child lacked awareness of other people's feelings, did not realise when others were upset or angry, was oblivi-ous to the effect of her behaviour on other members of the family, was very demanding of people's time, was difficult to reason with when upset, unknowingly offended people with her behaviour, did not respond to commands, and other similar questions. The parents had to respond with 0 (for 'not at all true'), 1 for 'quite or sometimes S E X 2 1 7
true' and 2 for 'very or often true'. The total from all twelve questions was then totted up. All the Turner's girls had higher scores than normal girls and boys, but the ones who were lacking the paternal X chromosome had more than twice the score of the ones lacking the maternal X chromosome.
The inference is that there is an imprinted gene somewhere on the X chromosome, which is normally switched on only in the paternal copy and that this gene somehow enhances the development of social adjustment - the ability to understand other's feelings, for example. Skuse and his colleagues provided further evidence of this from children who were missing only part of one X chromosome.17
This study has two massive implications. First, it suggests an explanation for the fact that autism, dyslexia, language impairment and other social problems are much commoner in boys than girls.
A boy receives only one X chromosome, from his mother, so he presumably gets one with the maternal imprint and the gene in question switched off. As of this writing, the gene has not been located, but imprinted genes are known from the X chromosome.
But second, and more generally, we are beginning to glimpse an end to the somewhat ridiculous argument over gender differences that has continued throughout the late twentieth century and has pitted nature against nurture. Those in favour of nurture have tried to deny any role for nature, while those who favour nature have rarely denied a role for nurture. The question is not whether nurture has a role to play, because nobody of any sense has ever gone on record as denying that it does, but whether nature has a role to play at all. When my one-year-old daughter discovered a plastic baby in a toy pram one day while I was writing this chapter, she let out the kinds of delighted squeals that her brother had reserved at the same age for passing tractors. Like many parents, I found it hard to believe that this was purely because of some unconscious social conditioning that we had imposed. Boys and girls have systematically different interests from the very beginning of autonomous behaviour. Boys are more competitive, more interested in machines, weapons and deeds. Girls are more interested in people, clothes and words. To 2 l 8 G E N O M E
put it more boldly, it is no thanks only to upbringing that men like maps and women like novels.
In any case, the perfect, if unconscionably cruel, experiment has been done by the supporters of pure nurture. In the 1960s, in the United States, a botched circumcision left a boy with a badly damaged penis, which the doctor decided to amputate. It was decided to try to turn the boy into a girl by castration, surgery and hormonal treatment. John became Joan; she wore dresses and played with dolls. She grew up into a young woman. In 1973 John Money, a Freudian psychologist, claimed in a burst of publicity that Joan was a well adjusted adolescent, and her case thus put an end to all speculation: gender roles were socially constructed.
Not until 1997 did anybody check the facts. When Milton Diamond and Keith Sigmundson tracked down Joan, they found a man, happily married to a woman. His story was very different from that told by Money. He had always felt deeply unhappy about something as a child and had always wanted to wear trousers, mix with boys and urinate standing up. At the age of fourteen he was told by his parents what had happened, which brought great relief. He ceased hormonal treatment, changed his name back to John, resumed the life of a man, had his breasts removed and at the age of twenty-five married a woman and adopted her children. Held up as a proof of socially constructed gender roles, he proved the exact opposite: that nature does play a role in gender. The evidence from zoology has always pointed that way: male behaviour is systematically different from female behaviour in most species and the difference has an innate component. The brain is an organ with innate gender. The evidence from the genome, from imprinted genes and genes for sex-linked behaviours, now points to the same conclusion.18
C H R O M O S O M E 1 6
M e m o r y
Heredity provides for the modification of its own machinery.
The human genome is a book. By reading it carefully from beginning to end, taking due account of anomalies like imprinting, a skilful technician could make a complete human body. Given the right mechanism for reading and interpreting the book, an accomplished modern Frankenstein could carry out the feat. But what then? He would have made a human body and imbued it with the elixir of life, but for it to be truly alive it would have to do more than exist.
It would have to adapt, to change and to respond. It would have to gain its autonomy. It would have to escape Frankenstein's control.
There is a sense in which the genes, like the hapless medical student in Mary Shelley's story, must lose control of their own creation.
They must set it free to find its own path through life. The genome does not tell the heart when to beat, nor the eye when to blink, nor the mind when to think. Even if the genes do set some of the parameters of personality, intelligence and human nature with surprising precision, they know when to delegate. Here on chromosome 2 2 0 G E N O M E
16 lie some of the great delegators: genes that allow learning and memory.
