Lead investigator Alan Sanders, a psychiatrist with the NorthShore University HealthSystem Research Institute in Illinois, never got the 1,000 brother pairs he was hoping to enroll. But the 409 pairs whose DNA he and his colleagues, including Bailey, were able to analyze made their work the largest linkage study ever of male sexual orientation. It took until late last year for them to publish
their findings, and by then genetic linkage studies had been eclipsed by more powerful genome-wide association studies. Still, the results are notable. In his talk at Lethbridge, Sanders saysthe study found that gay brothers share genetic markers on their X chromosome, replicating Hamer’s finding on the so-called Xq28 region, as well as a more robust, statistically significant finding on chromosome 8, replicating a follow-up finding from Hamer’s lab. The results, Sanders says, offer more solid evidence of genes’ influence on what makes people gay. But he also admits that genes likely play a lesser role in sexual orientation than the 40 percent estimate he gave me a decade ago. Now, he puts it around 30 percent, with lots of work still to be done on which genes may be involved and how. That means two-thirds of the answer will have to come from somewhere else.And that leads me back to the area that seemed most revealing a decade ago: the environment of the womb. Here, the evidence for sexual orientation being inborn has become only stronger, though not as conclusively as I expected.For many parents, the prenatal period is a black box — nine months of waiting, hoping for reassuring signals from the ultrasound machine and comforting nods from the technician. In reality, even identical twins like Patrick and Thomas can have very different prenatal experiences and often differ significantly in birth weight. (Patrick was 1 pound lighter than Thomas.) Crucially, the fetal brain is being organized, with sex hormones keeping it in its default female state or sending it along the male path. Researchers have suspected for some time that the exposure to sex hormones during this organizational period plays an important role in the baby’s ultimate sexual orientation. Here thereare all kinds of tantalizing clues.For instance, otoacoustic emissions, which are barely detectable sounds in the inner ear, are believed to have their levels set prenatally and are consistently different in males and females, suggesting the handiwork of hormone exposure. Butlesbians show male-type patterns.
Then there are people with a condition called congenital adrenal hyperplasia, or CAH, the result of having been exposed to unusuallyhigh levels of testosterone during the prenatal period.Adult womenwith CAHhave higher rates of lesbianism than the general population.