Normal-looking, furry, scurrying South American mice of several natural species appear, upon molecular examination, to violate most of the standard rules of mammalian sex determination.
In at least eight species of the mouse genus Akodon, from 10 percent to 60 percent of the females carry a Y chromosome in place of one of the two X chromosomes usually found in the cells of female mammals, report Hopi E. Hoekstra and Scott V. Edwards of the University of Washington in Seattle. In all but a few mammals, the presence of that Y chromosome would be synonymous with malehood.
Usually, XY females don't reproduce well, if at all. However, the genetically odd Akodon females have more pups than regular females do.