Among the X's blue genes are about 100 intimately involved in sperm production and one that allows the body to respond to male hormones produced by the testes.
Defective genes on the X easily lead to diseases in males because males lack a second, healthy X to compensate. Although a disease's linkage to the X can be obvious by the pattern of unaffected mothers giving birth to affected sons, the newly completed sequence will speed discovery of the specific genes involved and the development of novel therapies, scientists said.
Already, 307 diseases including hemophilia, colorblindness and muscular dystrophy have been linked to genes on the X. In fact, more than 10 percent of known diseases of heredity have been linked to genes on the X even though that chromosome has just 4 percent of all human genes.
But that does not mean the X is especially laden with problems. Other chromosomes probably harbor as many diseases, experts said, but the genetic linkages are harder to trace.
One of today's two reports focuses specifically on the bizarre mechanism by which females inactivate, or chemically mask, one X chromosome in each cell -- a trick to balance the output of genes on the X with the output from countervailing genes in male cells. That report, by Laura Carrel of Pennsylvania State University and Huntington Willard of Duke, affirms two unexpected facts.
First, it appears that strange sequences of DNA previously thought to be useless are key to the inactivation process, suggesting that similar stretches in other chromosomes may similarly be involved in regulating gene activity.
Second, at least 15 percent of inactivated X chromosomes in women routinely remain at least partly active, and in some women the figure can reach 25 percent. That means as many as 300 genes on women's Xs may be outperforming their counterpart genes in men.
That could explain some of the biological differences between the sexes, researchers said, including differences in responses to drugs and susceptibility to some diseases. And because the gene activation patterns apparently vary considerably from woman to woman, it suggests that women as a group are more variable than men.