A new scientific study at the University of Illinois has found that genetic manipulation in fruit flies can “turn” their homosexual behaviour on and off within a few hours.

Biologist David Featherstone said that his team had discovered a “gender-blind” gene in fruit flies which could be altered with drugs, turning them bisexual.

“The gender-blind mutant males treated other males exactly the same way normal male flies would treat a female. They even attempted copulation”, Featherstone said in an interview with the Chicago Sun-times newspaper.

The research, published online by the journal Nature Neuroscience, seems to show that in some animals homosexuality is not hard-wired.

In the case of the fruit flies, it would depend on how they interpret the scent of one another.

According to Featherstone, two “sensory circuits” were found in the flies’ brain, one causing heterosexual behaviour and the other causing homosexual behaviour.

“Based on our previous work, we reasoned that gender-blind mutants might show homosexual behaviour because their glutamatergic synapses were altered in some way. Homosexual courtship might be sort of an overreaction to sexual stimuli,” he said.

The Chicago-based scientist said that humans have a similar gene, but he added that it was not known yet if it had any influence in humans’ same-sex behaviour.

A “gay gene” issue has long influenced scientific and academic debate.

Last year British human rights activist Peter Tatchell criticised the “obsession” of the scientific community about finding any supposed gay gene.

“Why are scientists so preoccupied with what causes homosexuality, to the near-total exclusion of the factors that lead to heterosexuality?” he said.

“I don’t object to research into sexual orientation. It is the one-sided obsession that bugs me.

“The presumption seems to be that straightness is normal and therefore does not need explanation; whereas queerdom is a deviation from the norm and this requires investigation and answers.”