A recent study has found new evidence that male sexual orientation is genetic. Researchers discovered that genes on at least two chromosomes influence whether a man is gay or straight—and they are passed down exclusively from their mothers.
The study—which hasn't yet been published, but was presented at the annual meeting of the American Association for the Advancement of Science in Chicago—tested the DNA of 400 gay men and found that a region of the X chromosome called Xq28 affected men's sexual behavior, as did a portion of DNA on chromosome 8, however, the precise gene is not known. The X chromosome is inherited directly from mothers.
However, scientists also found that those genes were neither sufficient now necessary for gay men. Some gay men in the study did not inherit the Xq28 region. According to scientists who worked on the research:
Sexual orientation has nothing to do with choice. We found evidence for two sets [of genes] that affect whether a man is gay or straight. But it is not completely determinative; there are certainly other environmental factors involved.
In other words, genetics is not the whole story when it comes to something as complicated as sexuality.