Lots of gay men have a straight male twin. To be specific, the identical twin of a gay man, despite having the same genes, has only a 20 to 50 percent chance of being gay himself.
So we can’t say genes entirely decide who is gay. Nor can we expect scientists to produce tests for sexual orientation.
But researchers are getting closer to understanding the biology of male same-sex attraction through the growing science of epigenetics, tagging systems that mark genes to be turned on or off. Some of these tags, or “epi-marks,” can be passed between the generations or develop in the womb or first nine months of life. The best-known tag is a “methyl” group that binds to specific areas of the DNA, a process called “methylation.”
In a 2012 paper, scientists suggested that tags passed from father to daughter or from mother to son might influence a fetus’ sensitivity to testosterone in the womb, affecting the child’s sexual attraction as adults.
To test whether methylation was involved, other researchers then studied the methylation patterns of 37 pairs of male identical twins in which one was gay and the other straight — as well as in 10 pairs of gay twins. They found five regions of DNA that seemed linked to sexual orientation and identified tagging patterns that predicted sexual orientation, within the group, by almost 70 percent.
The team was careful to say, however, that they can’t predict sexual orientation in the larger population through this research.
It’s also important to note that the regions showing the methylation marks tied to same-sex attraction were not clearly linked to testosterone sensitivity in the womb. One gene was important for nerve conduction and another for immunity.
Finally, no one knows why identical twins can end up with different methylation patterns. The fetus’ location within the womb and how much maternal blood he receives could affect the outcome.
Another factor: men with older brothers by the same mother are likelier to be gay than men with older sisters or no older siblings. In fact, your likelihood of being gay increases by about 33 percent with each additional biological older brother. About 15 to 30 percent of gay men have older biological brothers.
One theory suggests that after delivering a boy, the mother produces antibodies to male-specific proteins which affect a male fetus during subsequent pregnancies. Tracing the effect to the womb is more plausible because it doesn’t seem to matter whether you ever lived with the older brother and half-brothers with a different mother to influence sexual orientation.
November 18, 2015
Christopher Nystuen, MD, MBA