Sexy Genes

Scientists have long believed that at the genetic level, men and women are pretty much the same. According to textbooks, only two of the 46 gene-carrying chromosomes in a human cell -- a pair known as the sex chromosomes -- are noticeably different in males and females. But at a genetics seminar last week at Jackson Laboratory in Bar Harbor, Me., this conventional wisdom took a beating. Participants cited evidence that there may be many more differences in male and female genes than previously thought. That revelation challenges assumptions about heredity held for more than a century.

Human chromosomes come in 23 pairs. One member of each pair is from a person's mother and the other from the father. Except for the two sex chromosomes, the members of each pair were thought to be interchangeable. It seemed that the way a chromosome functioned in the cell had nothing to do with whether it came from the mother or the father.

But new studies of genetic diseases suggest that the chromosomes from the two parents can play slightly different roles. For example, children who through a reproductive malfunction receive two copies of chromosome No. 7 from their mother and no copy from their father sometimes suffer from a severely retarded growth rate. Naturally occurring cases of a kidney cancer called Wilms tumor are caused by a missing chromosome. But in almost all cases, the missing chromosome was the mother's, not the father's.

"This is just absolutely mind boggling," said Judith Hall, a geneticist from the University of British Columbia. "It's a new way of thinking." And that new way may continue to reveal subtle differences in the genetic makeup of the two sexes.