It turns out that the "gay gene" is on the X chromosome, of which females have two and males have one.
Andrea Camperio Ciani and colleagues at the University of Padua, Italy, showed that the female relatives of homosexual men tend to have more children, suggesting that genes on the X chromosome are responsible. Now the team have shown that the same is true for bisexuality.
"The answer is remarkably simple: the same gene that causes men to like men also causes women to like men, and as a result to have more children."
The article has details about the research itself."