THE part of humanity’s genetic inheritance that most unites the sexes also explains many of the deepest differences between men and women, a groundbreaking new study has indicated.

The first map of the X chromosome — one of the two coils of DNA that determine gender — has revealed that Homo sapiens essentially boasts not one genome but two: the male and the female.

The findings, from an international research team led by the Wellcome Trust Sanger Institute near Cambridge, suggest that a powerful new genetic mechanism may underlie many of the characteristics that vary between the sexes.

Traits such as aggression and empathy, which are more common on average in one sex, could be influenced by the differing behaviour of the X chromosome in men and women, scientists said.

The X chromosome map will also shed important light on hundreds of genetic diseases, particularly conditions such as haemophilia and autism that mainly or exclusively afflict boys, and could lead to new methods of genetic screening and even cures.

In human beings, as in all mammals, gender is determined by which of the two sex chromosomes people inherit from their parents. Females have two copies of the X chromosome; males have one X and one much smaller Y chromosome. While the X chromosome is common to both sexes, a new analysis made possible by the detailed map has shown that it works very differently in each of them.

It was previously thought that in women one copy of the X chromosome was almost entirely switched off in every cell, to prevent duplication of its genetic instructions. This left only the handful of genes on the Y chromosome as a genetic barrier between the sexes.

The new research has shown that this spare female X is not turned off as completely as had been assumed. Up to 25 per cent of its genes in fact remain active, giving women a “double dose” that could have important effects.

“Our study shows that the inactive X in women is not as silent as we thought,” said Laura Carrel, of Pennsylvania State University, who led this element of the research. “The effects of these genes from the inactive X chromosome could explain some of the differences between men and women that are not attributable to sex hormones.”

Her co-author, Hunt Willard, of Duke University in North Carolina, said: “We now know that 25 per cent of the X chromosome — 200 to 300 genes — can be uniquely expressed in one sex relative to the other. In essence, there is not one human genome, but two — male and female.”

Many of these genes also vary in their pattern of expression from woman to woman, the researchers found. This means that women are less genetically similar to one another than are men, and may contribute to individuals’ behaviour or susceptibility to disease.

The X chromosome map, which is published today in the journal Nature and is freely accessible to researchers, also has profound implications for the study of hundreds of disorders in which genetics plays a part. When mutations arise on the X chromosome they are particularly likely to cause disease in males: while females have a back-up X that can usually compensate for the error, the Y chromosome carries no spare.

More than 300 conditions are already known to be caused or influenced by defective genes on the X chromosome, by far the highest total for any human chromosome. They include haemophilia, Duchenne muscular dystrophy, red-green colour-blindness and several kinds of mental retardation, and possibly genetic triggers for autism and infertility.

Segments of the X chromosome map were published on the internet as they were finished and these have already led to the identification of 43 new genes, including those that cause blindness and cleft palate.

“There are still 132 conditions we know are linked to the X, but for which we have got to find the causative gene,” said David Bentley, of the Sanger Institute. “Now we can make use of the finished sequence to find them. These discoveries will have a major impact on our understanding of many fundamental biological processes.”

The identification of X chromosome genes that cause disease has practical consequences, such as improving diagnosis, which can be important in selecting the best therapies.

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