One gene, one trait is so very rare. Many times it is multiple genes and polygenetic reactions are so difficult to predict. And this is all assuming the environment is the exact same. Two tall parents are never guaranteed to have a tall kid. It is more likely, but not a given.
Behavioral traits are a whole different ballgame.
Not only are they combination of multiple genes, they are probably swamped out by environmental effects. Herding is a behavioral trait. It is an instinctive behavior granted, but is a behavior nonetheless. If any breeder can guarantee me that a pup born from two advanced working dogs will have an advanced herding instinct is either lying or mistaken. I have a dog that I would breed, even though she doesn't have to work sheep for a living nor has she won lots of trial competitions.