There is a sex ratio of about 1:1 in most species, including humans, meaning that an approximately equal number of males and females are produced. For the first time, geneticists at Emory University in Atlanta, Georgia, have discovered a genetic mechanism delivering this balanced ratio, at least valid for fruit flies, as wrote in the article published in the November issue of PLoS Biology.

Wild fruit flies display the 1:1 sex ratio, but some lab lines deliver more females than males, and researches could only guess this was connected to X and Y sex chromosomes.

The team compared the DNA of male fruit flies that delivered 90% female offspring to the DNA coming from a strain producing a balanced number of each sex. The variation was placed in the X chromosome gene named Dox. Male individuals with Dox delivered healthy sperm with the X chromosome (that produces females), and somehow disabled the Y chromosome sperm carrying Dox, that's why most of the eggs were fecundated by X sperms, that gave females.

In a strain lacking Dox, the Y sperms (delivering males) were equally stronger than X sperms, producing a balanced sex ratio among offspring. The analyses also detected a gene named Nmy on another chromosome that inhibited Dox. Male individuals carrying both Dox and Nmy had a balanced number of male and female offspring, while in the case of losing Nmy, they produced more females.

"Although it's not clear whether these genes are present in other species, the finding could explain a widespread phenomenon--known as meiotic sex chromosome inactivation--in which the X and Y chromosomes are turned off during the production of sperm. This shutdown could be a method of thwarting Dox and thereby preventing the X chromosome genes from dominating the show", said lead researcher Tao Yun, evolutionary geneticist.

The findings "raise the possibility that Dox and Nmy may lead to the accumulation of genetic changes on the X chromosome", said Daven Presgraves, an evolutionary geneticist at the University of Rochester in New York state.

This could be a step towards new species by producing individuals that cannot successfully breed with any individual of the population. "The origin of new species could be an incidental byproduct of evolutionary arms races between selfish genes and their suppressors," said Presgraves.