I have not read anywhere that there is genetic marker of some sort. I am now sitting here pondering would it really make a difference to me if it were so called medically proven. Because I am such a skeptic and in medical field, I don't believe half the stuff I read anyway. I am especially suspect when I see certain medical conditions attributed more to African Americans yet I never see or have seen an AA with the disorder after being in the field for over thirty years. I will say I always leaned more towards choice up until about about 33 years ago. This child's mannerisms as a toddler was feminine. We all thought the same way. That boy is going to be gay. He is. Grew up with a mom and dad that presented themselves as their gender. His surroundings were heterosexual. If there is info out there suggesting a medical reasoning behind it, I have not come across it yet.