MrRING
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X marks the spot where women differ
Steve Connor
London - Genetic differences between men and women are more extensive and profound than previously believed, according to a study that unravels the complex chemistry of the sexes.
A detailed analysis of the "X" chromosome - the female sex chromosome - has revealed that women are genetically more complicated than men. The findings revealed that men have taken a genetic battering that has diminished the size of their own "Y" sex chromosome.
The battle of the sexes has its roots in a 300-million-year struggle between the X and the Y chromosomes which have vied with each other for influence over successive generations of males and females. On Thursday, scientists showed that it has been the X chromosome that has retained its physical integrity while the Y chromosome of men has dwindled in size and power to become a mere shadow of its former self.
The first comprehensive survey of the genes that are active on the X chromosome has revealed the dominant position that the female sex chromosome has gained in the long war of attrition with its male counterpart. One consequence is that boys are more prone to genetic diseases than girls.
While the Y chromosome has retained fewer than 100 working genes, the X chromosome contains more than 1 000 and can deploy them in a more intricate way in women. While women have two X chromosomes, men only have one, which they inherit from their mothers. The second sex chromosome of men is the Y chromosome of their fathers, which is thought to have evolved from a degenerate version of an ancient X chromosome as long at 300 million years ago.
Huntington Willard of Duke University in Durham, North Carolina, and one of the leaders of the study published in the journal Nature, said the findings show that when the X chromosome occurs in women it behaves so differently to when it resides in men that it has effectively resulted in the evolution of another human genome. "We now know that 25 percent of the X chromosome - some 200 to 300 genes - can be uniquely expressed in one sex relative to the other. In essence, therefore, there is not one human genome, but two - male and female," Willard explained.
Originally it was thought that one of the two X chromosomes in women was completely turned off or inactivated so that the female body did not become inundated with twice as many X-chromosome genes as men. However, the latest study paints a far more complex and subtle picture. Laura Carrel of Penn State College of Medicine looked at inactivity levels in the X chromosomes of 40 women.
She found that 65 percent of X chromosome genes were totally inactivated, 20 percent were inactivated in some samples but not others, and 15 percent escaped inactivation altogether.
"Our study shows that the inactive X in women is not as silent as we thought. The effects of these genes from the inactive X chromosome could explain some of the differences between men and women that aren't attributable to sex hormones," Carrel said.
In other words, the physical and emotional differences between men and women may run deeper than those caused simply by hormones. The study showed, for instance, that the 15 percent of genes that are active on the "inactive" X chromosome are present at higher levels in women than they are in men."These differences should be recognised as potential factors for explaining normal differences between the sexes but also gender differences in how certain diseases are manifested, progress and respond to treatment," Carrel said.
Girls have two X chromosome so defects in one can be corrected by the other. However, because boys have only one X chromosome they suffer from inherited disorders when the X chromosome is damaged, such as haemophilia, colour blindness and Duchenne muscular dystrophy.Mark Ross, project leader of the X chromosome study at the Wellcome Trust said: "We can see the way evolution has shaped the chromosomes that determine our gender to give them their properties." - The Independent
http://www.iol.co.za/index.php?set_id=1&click_id=588&art_id=vn20050318081014124C694218
Other aspects of this story:
http://www.nzherald.co.nz/index.cfm?ObjectID=10115838
http://www.azcentral.com/news/articles/0317x-chromosome17.html