BOTH genetics and the dogs individual personality in addition to it's enviornment which includes the owners ability as a responsbile dog owner plays a factor in the dogs personality. Example are Mastiff's. They are genetically bred to be guard dogs. This guarding instinct varies on the breed and personality of the dog. English Mastiff usually use their size and sound of their bark to intimidate rather than launch into an attack. Moving a 200lb body is hard work so if they can accomplish they same outcome, to deter an intruder, without aggression they usually will. Now the Dogue de Bordeaux, due to it's past history as a fighting dog, can be more aggressive in their guarding instinct. Genetically they tend to have more dominant personalities than the English Mastiff. Same sex aggression issues are inherent in the breed. The owner must take care when they are around other dominant dogs.
While the issue of dog aggression is complex, all factors come into place and must be realized. Disregarding genetics as a factor, only leads to the downfall of the dog.