Well, we have two sons, both raised the same way and one is an AF Pilot, the other "was" going to join the Navy until he had a psychotic break and developed Schizoaffective disorder. They now know that there are definite genetic components to these illness, but that, unlike some illnesses like Huntington's, the genetics alone are not the whole story. Lots of debate (and thankfully study) on what the other factors might be. Some think it could even be viral while still in the womb, others feel it could be head trauma that "triggers" someone pre-dispoded genetically. Our son, had two instances in younger years where he suffered pretty nasty head injuries and we have long suspected that might have been his "trigger."
Thankfully, he is doing very well now with the right medication and therapy. In fact he is in Civil Air Patrol and just got promoted to Senior Member First Lt. Probably not a big deal for most folks, but a huge deal for him!