ONE could be forgiven for thinking that the fuss about the human genome was over. Five years ago, the politicians gave the beaming scientists a collective slap on the back and champagne corks were popped. But after the hangovers cleared, the scientists had to go back to their labs and do the job properly. The “draft” sequence they had released was just a landmark along the way.

Gradually the details of each human chromosome have been emerging. A chromosome is a set of instructions, in the form of genes, that are used in creating an organism. This week scientists have reported in the journal Nature that they have finally finished sequencing the X chromosome.

The X is one of two chromosomes that determine sex. Women have two Xs, while males have one X and a Y. These sex chromosomes originated between 230m and 300m years ago, evolving from the same non-sex chromosome in some ancestral mammal species. Over the course of evolution, the Y has become severely eroded by mutation, and today is stunted, with only one hundred genes. The new research reveals that the X has over 1,000 genes.

Because women have their sex genes written in duplicate, they always have a second chance of a correct version. This means they are less prone to sex-linked genetic defects. Such defects are largely why the birth-rates between boys (49%) and girls (51%) are not equal.

New work on the X has now revealed more about how differences between the sexes are determined at a genetic level. One route is the way in which this second dose of X genes is regulated. What was already known was that in each individual female cell, one of the two X chromosomes was randomly inactivated. This is so that female cells do not have twice as much genetic activity as male cells.

However, work by Laura Carrel, at Pennsylvania State University, and Huntington Willard, at Duke University in North Carolina, now shows that this inactivation is not complete. For a start, 15% of genes that were expected to be switched off (so that they do not produce proteins), actually escape inactivation.

This is a big surprise. And it means that in female cells a certain proportion of activated genes exist in higher concentrations than was once thought. The extent that this matters will very likely depend on the gene itself. As of yet, though, nobody knows what impact this has on women. The study also found that another 10% of the genes on the X are sometimes inactivated, and sometimes not. So some women might have a single dose of these genes while others will have a double dose. Furthermore, the pattern in which these 10% of genes activate seems to be different in every woman the authors have looked at so far.

The authors examined the genomes of 40 women and found that in each case, the pattern of expression of these genes was unique. Such “expression heterogeneity” was unsuspected, but is exciting because it suggests that women could be more genetically variable than men.

This kind of work is important for two reasons. The most important, though obvious, reason is to help us better understand sex-linked genetic defects in men after birth. These are important in areas such as cancer, fertility and mental retardation (males have a higher incidence).

A perhaps slightly less important, though more unexpected, reason for interest is that this work offers several new ways of exploring the genetics behind the difference between the sexes. The president of Harvard recently stirred up a storm of protest by speculating that differences in aptitude, rather than discrimination, may explain why fewer women achieve excellence in science. There is certainly no “gene for science”. But this work tells us that somewhere between 200 to 300 genes can be uniquely expressed in women. Hot pursuit of these genes now seems likely. More surprises lie ahead.