Its a good question - it may be b/c we are each unique even though we pick up general tendencies and similarities from our parents and ancestors - still not exactly the same though. Genes mya leave us predisposed to tmj due to structural developent patterns but then the dentistry each receives may be different. I guess one would have to look at their overall developmental and dental history - wisdom teeth, braces, missing teeth, to see where the variance lie. My mom had jaw problems but wore dentures starting in her 40's yet none of my sibs have any problems - yet I have an extensive dental history including trauma to the face so for me I am pretty sure its not really hereditary.