(U-WIRE) DURHAM, N.C. Although women may not be scientifically proven to differ from men in intelligence or motivation, researchers at Duke have discovered women are more complex – genetically, at least.

In a study published in Nature, researchers from Duke and Pennsylvania State University found surprising levels of genetic variation on the X chromosome, an area that has not been fully explored until now. The findings suggest women and men are more different than originally believed.

“When considering this additional number of genes as well as the variability in gene expression on the X chromosome, women are more complex than men, in that sense,” said Huntington Willard, director of Duke’s Institute for Genome Sciences and Policy.

The human genome, the complete set of genes within an individual, is comprised of 23 pairs of chromosomes, one pair of which is responsible for determining gender. These two chromosomes, called sex chromosomes, are designated by the letters X and Y. Males have an X and Y, while females have two X chromosomes.

Genes, a DNA sequence that encodes the recipe to create proteins in the body, are located on chromosomes. Because women have two sets of X genes, female cells choose to de-activate one copy of the X chromosome in a process called “silencing.” This is to avoid any harmful effects that would result in double expression of such genes.

Willard and Laura Carrel, a former postdoctoral fellow in Willard’s lab and now assistant professor of biochemistry and molecular biology at Pennsylvania State University, said they were surprised by their finding that at least 15 percent of the genes on the X chromosome escape silencing and another 10 percent show variable degrees of expression among women.

The results of the study, however, not only reveal the degree of genetic differences between the sexes, they also show differences among women themselves. While one woman may have her copy of a particular X-linked gene expressed, another woman may not. These un-silenced or partially silenced genes on the X chromosome comprise more than 1 percent of the entire genome, accounting for more than 200 to 300 more expressed genes in women than in men.

“What our finding superimposes upon the characteristic differences between men and women is how much of that variation is present from one female to another,” Willard said.

Recognizing this variation is important for medicine, as there are a large number of diseases that are much more common among women than men. “We’ve known that a lot of the differences in gender-based medicine are due to hormonal effects and cultural effects,” Carrel said. “What our research says now is that we should look in addition at the genetic differences between men and women.”

“We went into this from a basic perspective to understand the basis of silencing events,” said Carrel, “but we’re finding important results that have medical implications.”