Researchers at the Texas A&M University have identified the mechanism behind sex determination in fruit flies.
The study, led by James Erickson and Jerome Quintero at the university, studied fruit fly Drosophila melanogaster to reach their conclusions.
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.
However, other animal groups have evolved different systems.
Previous studies in the fly have 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, the latest study has indicated 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.
The study has been published online in the open-access journal PLoS Biology.