U.S. researchers say they've come up with a formula that can show someone's sexual orientation by looking at genetic changes.
It's a controversial idea, and they have not made public the details of what they did. But the research, being presented at a meeting of genetics experts, suggests a variety of factors come together to help determine whether someone is gay or straight.
"To our knowledge, this is the first example of a predictive model for sexual orientation based on molecular markers," said Tuck Ngun, a researcher at the David Geffen School of Medicine of the University of California, Los Angeles, who led the study.
Other experts said Ngun may be going too far in saying he can predict someone's sexual orientation by looking at his or her genes. His study group was very small.
The idea that sexuality can be found in the genes isn't at all new — Dean Hamer of the National Institutes of Health reported in 1993 that he had found a batch of genes linked with homosexuality, and researchers have reported a variety of genetic findings since then. One thing they agree on: there is no single "gay gene".
Genetic changes can be handed down from generation to generation, or they can be made as a part of living life, from the moment a child is conceived through adulthood. These are called epigenetic changes and while they don't change the underlying code, they can alter how a gene is expressed — how it works.
"The observed epigenetic changes, particularly if from blood DNA, unlikely determine the complex behaviors, such as sexual orientation," said Dr. Peng Jin, a professor of human genetics at Emory University.
Ngun told an American Society of Human Genetics meeting in Baltimore that he looked at epigenetic changes called methylation in 47 pairs of male twins. Identical twins have the same underlying DNA, but the epigenetic changes can make big differences in what happens to them later in life.
In 37 of the twin pairs, one brother was homosexual and the other wasn't. In 10 pairs, both brothers were.
Ngun and his colleagues came up with a computer algorithm, a formula, that suggested that patterns of methylation in nine regions were associated with sexual orientation with 67 percent of the time.
"Sexual attraction is such a fundamental part of life, but it's not something we know a lot about at the genetic and molecular level. I hope that this research helps us understand ourselves better and why we are the way we are," Ngun said.
Other researchers said the idea would have to be tested in many, many more people to see if the effect is real. "Without validation of the result in an independent data set it is not really possible to know whether there is any substance in this claim," said Gil McVean, a statistical geneticist at Britain's University of Oxford.
"My gut feeling it that, as the complete story unfolds, the association may not be quite as simple as the summary (abstract) and press release suggest. The important thing to note however is the mounting evidence that homosexuality is a perfectly normal trait segregating in human populations," added genetics professor Darren Griffin of the University of Kent.
But Dr. Margaret McCarthy, who studies the developing brain at the University of Maryland School of Medicine, said epigenetic changes could happen while a fetus is developing.
"Developing male fetuses produce very high quantities of testosterone during the second trimester and this directs psychosexual development along masculine lines, a component of which is preference for females as sexual partners," McCarthy said in a statement.
"This study provides a major step forward in our understanding of how the brain can be affected by factors outside of the genome. It is also possible that the experience of being a homosexual or a heterosexual has itself impacted the epigenetic profile. But regardless of when, or even how, these epigenetic changes occur, their findings demonstrate a biological basis to partner preference."
According to Gallup, about 3.8 percent of the adult population identifies as lesbian, gay, bisexual or transgender.