Something not often recorded by medical science is a gene which affects human sex characteristics. It only occurs in a miniscule number of humans across the world, a few dozen at most, and is not known to occur in any circumstances to people who are prepubescent, but its effect on the human body is striking.
During the full moon, the bodies of those with the gene respond to the moon's light respond by producing massive amounts of hormones that dramatically affect sex characteristics. For those who are men the rest of the time, the body stimulates massive growth to the breast tissue, reduces muscle mass, modifies its proportions from masculine to feminine, moves the testicles inside the body and redevelops them into ovaries, splits the scrotum into labial lips, and causes the penis to recede until it forms a clitoris. Those who are female the rest of the time experience intense increases in muscle mass, body hair growth, their proportions shifting from feminine to masculine, the sealing up of the vagina, ovaries reforming into testicles, and dramatic growth of the clitoris into a penis.
The condition, if the affected person does not pursue any sexual activity, lasts 12 hours. If they do partake in sexual activities, be they sex, masturbation or anything else, the condition lasts 12 hours longer each time it is done. It is said, though it has not been proven, if two people with the condition were to have sex, its effects on both of them would be made permanent.