WEDNESDAY, Oct. 22, 2003 (HealthDayNews) -- Researchers say they've discovered genes that may help determine sexuality in the early days of life, weeks before hormones kick in and force fetuses to develop into males or females.
In some cases, the genes may go one way -- affecting the development of the brain -- while the hormones head in the opposite direction, says study co-author Dr. Eric Vilain, an assistant professor of human genetics at the University of California at Los Angeles. If the brain and hormones are at cross-purposes, that could shed light on why transgendered people feel as though they are trapped in the body of the wrong sex, he explains.
The findings "may explain the mechanism of gender identity, why we feel that we're male or female," Vilain adds. "What we're saying, and we're not the only ones to say this, is that [hormones] are not whole story. There also might be genetic influences independent from hormones."
Researchers have long assumed the hormones estrogen and testosterone are responsible for gender development in humans, Vilain notes.
At about six weeks of life, the hormones kick in and create testes or ovaries in humans. Before that time the fetus is physically sexless. The hormones also supposedly affect brain development, which is slightly different in males and females.
In his study, Vilain and his colleagues examined mice before they reached a similar stage of development. They wanted to see if certain genes in the mice "switched on" in accordance with their gender, which only became obvious later during physical development.
The results of the study appear in the October issue of Molecular Brain Research.
The researchers found 54 genes that seem related to gender. Eighteen were produced at higher levels in males; the 36 others were produced at higher levels in females.
In addition to possibly explaining why transgendered people feel as though they're men trapped in women's bodies (or vice versa), the findings could lead to genetic testing to help parents decide how to raise babies who have sex organs of both genders, Vilain says. Parents often have problems deciding whether these children should be raised as boys or girls and whether they should have surgery to "correct" their bodies.
Vilain thinks sexual orientation is a different matter, however, and he's not as certain these genes could contribute to homosexuality.
Bruce McEwen, head of the neuroendocrinology laboratory at Rockefeller University, says the ideas behind the study aren't new, but the research is still "very worthwhile."
But he cautions against assuming genes alone are responsible for different types of sexuality. "It's a nature-nurture type of thing in the extreme," he says. "Environment will play a role, genes will play a role. It would be dangerous to be simplistic."
The next step for researchers is to tinker with the genes of mice and see how they behave, Vilain says. Male and female mice act differently, with males being more aggressive. (There's no word yet on whether they're less likely to ask for directions, too.)