NOW here’s a brain bender: an extra set of female genes appears to make males more masculine. The surprising discovery suggests that sex chromosomes play a role in directing behaviour that extends beyond the effects of hormones.

“The predominant idea is that the difference between male and female behaviours is down to hormones,” says Emilie Rissman at the University of Virginia in Charlottesville. This starts early in life – male fetuses are exposed to testosterone from 4 weeks old, while females are not.

To find out if sex chromosomes play a role in sex-specific behaviours beyond dictating which hormones are present, Rissman’s team took advantage of a mutation in mice that causes the sex-determining region of the male Y chromosome to jump to a non-sex chromosome. The mice are male but have two X chromosomes.

While these XX male mice had the same level of testosterone as normal XY mice, they displayed more masculine sexual behaviours – mounting females more often and ejaculating more frequently.


Male mice with two X chromosomes mounted females more often than normal XY males did

To confirm that the differences were a result of a hidden factor on the X chromosome and not the lack of a Y chromosome, the team compared XY male mice with XXY male mice, which carry an extra X chromosome. Sure enough, the XXY mice also showed more male sexual behaviours (Hormones and Behaviour, DOI: 10.1016/j.yhbeh.2012.02.003).

The finding tallies with research that suggests a female’s second “inactivated” X chromosome may actually express a quarter of its genes. These could alter the expression of genes on other chromosomes, and might be partly responsible for behavioural differences between the sexes, says Rissman.

“The extent to which these findings are generalisable to humans remains to be seen,” says William Davies at Cardiff University, UK. However, the idea may provide an explanation for evidence that XXY men have more sex than men with the regular XY.

If Rissman’s team can identify a region of the X chromosome that is linked to sexual activity, its protein products could be a target for libido-boosting therapies.