Women Are Very Much Not Alike, Gene Study Finds

Scientists have found genetic evidence for what some men have long suspected: It is dangerous to make assumptions about women.

The key is the X chromosome, the feminine sex chromosome that all men and women have in common.

In a study published today in the journal Nature, scientists said they had found an unexpectedly large genetic variation on the X chromosome among women. The findings were published in conjunction with the first comprehensive decoding of the chromosome, which appeared in the same journal.

Females can differ from each other almost as much as they do from males in the behavior of many genes at the heart of sexual identity, researchers said.

"Literally every one of the females we looked at had a different genetic story," said Duke University genetics expert Huntington Willard, who co-wrote the study. "It is not just a little bit of variation."

The analysis also found that the obsessively debated differences between men and women were, at least on the genetic level, even greater than previously thought.

As many as 300 of the genes on the X chromosome may be activated differently among women than among men, said molecular biologist Laura Carrel at Pennsylvania State University College of Medicine, the other author of the paper.

The newly discovered genetic variation among women might help account for differing gender reactions to prescription drugs and the heightened vulnerability of women to some diseases, experts said.

"The important question becomes how men and women actually vary and how much variability there is in females," Carrel said. "We now might have new candidate genes that could explain differences between men and women."

All told, men and women may differ by as much as 2% of their entire genetic inheritance, greater than the hereditary gap between humankind and its closest relative -- the chimpanzee.

"In essence," Willard said, "there is not one human genome, but two -- male and female."

Scientists estimate that there may be as many as 30,000 genes in the chemical DNA blueprint for human growth and development known as the human genome.

The genes are parceled out in 23 pairs of rod-like structures called chromosomes contained in every cell of the body.

The most distinctive of the chromosomes are the mismatched pair of X and Y chromosomes that guide sexual development.

Until now, researchers considered the shuffle of sex chromosomes at conception a simple matter of genetic roulette.