[
Science,
1995]
Sex has lots of advantages, as the number of species that indulge in it shows. But it also poses a potentially lethal problem. Most species use distinct X and Y sex chromosomes to determine who develops as female and who as male-and the female generally has more copies of the X chromosome than the male. But the X chromosome contains many genes needed equally by males and females, threatening females with what could be a lethal excess of X-chromosome gene products-or males with an equally serious deficiency. Researchers have known for decades that humans and other sexually reproducing species survive because of a correcting mechanism known as "dosage compensation" that equalizes the expression of X-linked genes between the sexes. But only now are they beginning to figure out how