[sacrosancty]

Heritability of traits is a really messy and difficult problem. Sere, you can breed tameness into wild foxes, but that doesn't mean you can easily breed more tameness into already tame foxes/dogs. There might be some low-hanging fruit that's easy to exploit and after that, it's mostly random chance and the environment that determines things. But that again depends on what the environment is like - if all dog owners routinely raise their puppies in the same extreme way, their inherited differences might either be invisible because none get the chance to express them, or even more significant because that's the only factor left that can sort out how they respond to their environment.

[jamesash]

One can start with an already tame population of foxes/dogs and select for aggression. Aggression would be a useful trait in a breed desired for protection/guarding.

[sacrosancty]

People have been doing that and yet somehow those aggressive breeds still frequently produce harmless puppies, so it's not obvious that breeders have actually achieved it. Maybe dogs have been domesticated for so long that we can't undo their behavior with just a couple of hundred years. Or maybe we can. It's hard, not obvious.