( The first comprehensive survey of gene ...)Variation in women's X chromosomes may explain differences among individuals, between sexes

The results may have important implications for understanding the differences in traits among women and between males and females, in terms of both health and disease, said Huntington Willard, Ph.D., director of the IGSP and the study's senior author. The findings also offer new insight into the basis for well-established differences between the sexes, he said.

Willard said that the extensive variation in gene activity in the sex chromosomes means that, in essence, there is not one human genome, but two -- male and female.

"We looked at the X chromosomes of 40 women and every one of them had a unique pattern of gene expression," Willard said. "All of that variation is completely unique to women. The X chromosomes of males are all the same in this regard."

Willard and study co-author Laura Carrel, Ph.D., of Penn State, reported their findings in the March 17, 2005, issue of Nature. The National Institutes of Health supported the research. In the same issue of Nature, more than 250 researchers including Willard and Carrel, reported the complete DNA sequence of the human X chromosome.

In animals, sexual identity is governed by sex chromosomes. In humans and other mammals, males have one X and one Y chromosome, while females have a pair of X's.

Many genes on the male Y chromosomes have been lost over evolutionary time, leaving the chromosome with fewer than 100 functional genes. In contrast, the X chromosome -- present in at least one copy in both sexes -- encodes more than 1,000.

More than 45 years ago, researchers discovered that genes on one copy of the female's X chromosome are switched off, a modification known as X inactivation
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Source:Duke University Medical Center