I was once engaged to a man who had been tested by a doctor and found to have YYY chromosomes.  The doctor had become suspicious because he was trying to sew up my fiance's slashed palm, and all his needles kept bending because my fiance's skin was so thick.  The doctor later wrote a paper in the 1980s about my fiance.  In case you're wondering, he was a police officer and served many years in Vietnam.  And, yes, sitting still in his seat at school was difficult for him as a child, thus he didn't get much further than a couple of years of community college (where he studied acting).  He resembled Lee Majors (the actor in "Six Million Dollar Man" TV show).  He was highly intelligent, highly disciplined, and understood people really well.  I personally believe that if his son were tested, they would find that having YYY chromosomes is a genetic trait that is often inherited.  It may even be a sign of a particular species of human male.