A genetic defect, what evidence is there for that?
There are physical differences, in the brain structure etc (ie it resembles the brain of a straight member of the opposite sex), and the way they react to different pheromones among other things.
But iirc they don't know if it's down to genetics, hormones in the womb, hormones after the womb, or a combination of these and other factors.
*i ain't going near the naming of what ever you lot want to classify this as. as a defect, anomaly, whatever, with a 10 foot barge pole.