In mammals, male or female development depends on the presence of the Y chromosome, which is only found in males because it includes masculinizing genes. But other animal groups have evolved different systems.
James Erickson and Jerome Quintero at Texas A&M University studied the mechanism of sex determination in the fruit fly, Drosophila melanogaster.
Previous studies in the fly suggested that it was the ratio of X chromosomes (the “female” chromosome, of which there are two copies in a female fly, and just one in a male) to the non-sex chromosomes (the autosomes) that determined the sex of a fly embryo.
However, this new paper indicates that rather than being dependent on the ratio, it is the number of X chromosomes that is important. Sex is determined during a very specific and short stage in embryo development, and only two X chromosomes can produce enough of a signal to feminize the embryo during this window of opportunity.
Citation: Erickson JW, Quintero JJ (2007) Indirect effects of ploidy suggest X chromosome dose, not the X:A ratio, signals sex in Drosophila. PLoS Biol 5(12): e332. doi:10.1371/journal.pbio.0050332