Several people have commented that they want a purebred dog partly because they can be assured of a certain temperament. Yet whenever the subject of "aggressive breeds" comes up everyone says "it's not the breed it's the trainer." That just seems contradictory to me.
There are specific traits in the various breeds which are selected for through artificial selection. All dogs have behaviors which are "dog" behaviors and which have been passed on through many, many generations, going back to their wolf ancestors. All dogs possess many of these behaviors to a greater or lesser degree than others. For instance, all dogs are capable of biting, defending, retrieving, (because it's part of hunting and animal and bringng it back to consume) stalking, chasing, grabbing, protecting. Dogs are dogs. There is no gene for aggression per say (to date). However, developmental onset and off sets are varied between breeds from artifical selection of certain traits. With that, predatory motor patterns are varied and stronger in some than others. What goes on in the brain chemistry, among other factors IS genetic. So, the predisposition of certain traits is genetic, although the trait itself is not.
We all know that Pit Bulls have been selected for to fight other animals. Greyhounds have been selected for running fast. Running is a trait that all dogs possess. But Greyhounds have been selected for running faster than other dogs. Blood hounds have been selected for being especially good at tracking a scent. All dogs can track a scent, but Bloodhounds have been selected to do it better.
Temperament is something the dog is born with. Temperaments do vary, as we all know. They do in every animal. However, compensations can be made with nurture and environment to some degree. Some dogs will
tend toward more aggressiveness. But that does not mean that that trait itself is hereditary. And it can be modified to some degree. If my Doberman had been mistreated or unsocialized, he may have turned out to be more aggressive than a Labrador, (for example). But a Labrador can still become aggressive with bad handling because it's a dog thing, not a breed thing. However, their thresholds are probably very different. The Lab may be able to tolerate more aversive stimuli than a Doberman because of the wiring in the brain.
Dogs with unknown parentage, unknown selection are more of a crap shoot. Purebred dogs whose ancestory can be traced are more homegenous, more the same.
I'm also disturbed by how many people assume that all shelter dogs have "issues" . . . that they're aggressive or not housebroken or bad with kids. While this is the case with some shelter dogs, many are dropped off because the owner "is moving" or "has too many dogs" or "it was my daughter's dog and she went to college." And many are simply lost dogs--strays.
Don't let it disturb you. Anyone with half a brain wouldn't make such rash generalizations. However, it is more of an unknown in many cases than going to a reputable breeder for a dog whose parentage one knows and whose breed traits are known. You don't know if the partents or grandparents of a shelter dog had hip dysplasia, unless there is a record. You don't know what temperament a puppy has without some ancestoral record. Testing helps, but it's only one part of the equation. I realize that many, many wonderful dogs exist in shelters. That's only logical. The point I'm making is that it is more of an unknown, a little more of a guessing game than a purebred dog whose ancestors are traceable and who comes from a reputable breeder.