Researchers say they have found that one gene is apparently by itself enough to create patterns of sexual behavior in fruit flies, a kind of master sexual gene that normally exists in two distinct male and female variants.
In a series of experiments, the researchers found that females given the male variant of the gene acted exactly like males in courtship, madly pursuing other females.
Males that were artificially given the female version of the gene became more passive and turned their sexual attention to their own gender.
"We have shown that a single gene in the fruit fly is sufficient to determine all aspects of the flies' sexual orientation and behavior," said the paper's lead author, Barry Dickson, senior scientist at the Institute of Molecular Biotechnology at the Austrian Academy of Sciences in Vienna. "It's very surprising.
"What it tells us is that instinctive behaviors can be specified by genetic programs, just like the morphologic development of an organ or a nose."
The findings published today in the journal Cell are certain to prove influential in debates about whether genes or environment determine who we are, how we act and, especially, our sexual orientation.
Experts said they were both awed and shocked by the findings.
"The results are so clean and compelling, the whole field of the genetic roots of behavior is moved forward tremendously by this work," said Dr. Michael Weiss, chairman of the department of biochemistry at Case Western Reserve University. "Hopefully, this will take the discussion about sexual preferences out of the realm of morality and put it in the realm of science."
He added: "I never chose to be heterosexual; it just happened. But humans are complicated. With the flies we can see in a simple and elegant way how a gene can influence and determine behavior."
The finding supports scientific evidence accumulating over the past decade that sexual orientation might be innately programmed into the brains of men and women. Equally intriguing, the researchers say, is the possibility that a number of behaviors - hitting back when feeling threatened, fleeing when scared or laughing when amused - might also be programmed into human brains, a product of genetic heritage.
"This is a first - a superb demonstration that a single gene can serve as a switch for complex behaviors," said Dr. Gero Miesenboeck, a professor of cell biology at Yale University.
Dickson, the lead author, said he ran into the lab when an assistant called him on a Sunday night with the results. "This really makes you think about how much of our behavior, perhaps especially sexual behaviors, has a strong genetic component," he said.