Head of the Human Genome project talks on Life Site about this question.
As Dr. Collins would agree, environment can influence gene expression, and free will determines the response to whatever predispositions might be present.
Dr. Collins succinctly reviewed the research on homosexuality and offers the following:
“An area of particularly strong public interest is the genetic basis of homosexuality. Evidence from twin studies does in fact support the conclusion that heritable factors play a role in male homosexuality. However, the likelihood that the identical twin of a homosexual male will also be gay is about 20% (compared with 2-4 percent of males in the general population), indicating that sexual orientation is genetically influenced but not hardwired by DNA, and that whatever genes are involved represent predispositions, not predeterminations [emphasis added].”
The heritability estimates for homosexuality is substantially lower than General Cognitive Ability, Extroversion, Agreeableness, Conscientiousness, Neuroticism, Openness, Aggression and Traditionalism!
Dr. Collins noted that environment–particularly childhood experiences–as well as the role of free will and choice affect us all in profound ways. As researchers discover increasing levels of molecular detail about inherited factors that underlie our personalities, it’s critical that such data be used to illuminate the issues, not provide support to ideologues.