PLoS One 3, e2282 (2008)

In evolutionary terms, homosexuality might be a detrimental trait that stops people from passing on their genes. But population geneticists have successfully modelled several theoretical explanations for its maintenance. A trio in Italy has now come up with a simple model that involves just two genes.

At least one of these genes, say the University of Padua's Andrea Camperio Ciani and his co-workers, must be on the X chromosome and act to increase fitness when expressed in females. These conditions produce a population within which a small proportion of individuals is gay and in which this proportion remains stable over time.