Scientists announced this week they had not only located a gene that determines sex in developing mosquito embryos but when they tweaked it female offspring were born with male genitals.

The experiments are significant because only female mosquitoes suck blood, and when they bite someone with a disease such as dengue or yellow fever, spread the infections. These diseases infect about 100 million people a year.

If the novel idea works outside the laboratory it could significantly reduce wild mosquito populations because there would be fewer females to breed.

"Genetic control methods that introduce a male bias to reduce mosquito populations are attractive and potentially effective measures to reduce the incidence of mosquito-borne disease," the US and Chinese scientists wrote in the journal Science Express.

Until now scientists have found it difficult to locate the exact genes that determine sex in mosquitoes because they are located in a tricky region of the genome where lots of sequences repeat themselves.

Using sophisticated genetic technology, the researchers pinpointed the gene that makes males male in the type of mosquito that transmits dengue, Aedes aegypti. They called the gene Nix.

When they injected Nix into a batch of developing embryos more than two-thirds of the females developed a combination of male genitals and testes.

University of Sydney medical entomologist Cameron Webb said while the results were interesting, there were several hurdles to overcome before the technique could be used to control mosquito populations in the wild. The Nix gene is found only in males and it would be almost impossible to inject it into wild embryos, as the scientists did in the laboratory.

"Genetic modification of mosquitoes is something that will eventually be part of managing mosquito-borne diseases, but I don't think it'll be a silver bullet to solve all our problems," he said.

Other research groups have modified mosquitoes so their offspring are mostly male, have shorter lifespans or are less susceptible to pathogens.