I agree that personality is entirely dictated by the personality of the parents. The black berkshires that I got were bred exclusively as feeders. They were bred to grow quickly and compete hard for food and water and be very very tough animals.
You would have to see these rats in order to believe them. They are enormous and they are much much stronger than the pet grade stock that I had been working with. I am entirely convinced that if something ever happened and these rats got loose they would replace the entire wild rat population in the area in a matter of no time.