Studies made on fruit flies (photo) seem to suggest that the most attractive males carry genes that can prove to be "not so good" for every gender of the offspring.

The researchers have noticed that mating with the "fittest" partner actually can result in dramatically lower rates of reproductive success in the subsequent generations. This seems to be linked to masculine or feminine traits expressed through genes.

These discoveries challenge many aspects of the classical evolution theory: on average, the lowest quality couple produced the best offspring after few generations while the highest quality individuals produced the worst offspring.

The research team at Queen's University measured the inheritance of "fitness" (quality and number of offspring) comparing samples of low and high quality males and low and high quality females to see which is the effect of sexual selection, through which the genders compete for high quality sexual partners.

Through sexual selection, high quality genes are passed to the offspring, this measuring to reproductive success. But if sexual genes that benefit one sex are not fitting for the other, their prevalence leads to a situation when "…females who seek out high-fitness males will find that they produce high quality sons, but this will have detrimental effects on their daughters," says Dr. Adam Chippindale, a Canada Research Chair in evolutionary genetics.

Sexually antagonistic genes exert powerful sexual effects and are mostly grouped in the X chromosome, which the sons inherit only from their mothers. "Sexually antagonistic genes, may be harmful or compromise fitness by reducing fertility of the opposite sex" says Chippindale.

"If there are genes which have effects that feminize or masculinize each individual, then the expression of a wide variety of such genes will produce a continuum of gender. People wonder why there is so much gender identity diversity in the human populations - this kind of mechanism may help us get a handle on that."

Scientists suspect that these effects are more widely spread in the animal kingdom, and this phenomenon could explain a lot of sexual behaviors, like homosexuality. Too "macho" fathers could produce lesbian or very masculine daughters, while too feminine mothers could produce gay or effeminate sons.