More than half of all dogs in the United States are mixed breeds

More than half of all dogs in the United States are mixed breeds, and but even if your mutt has purebred grandparents, you can't necessarily predict its nature. Scott and Fuller studied breed differences in American Cocker Spaniels and Basenjis (as well as other breeds), such as reactivity to a doorbell, problem-solving, and spatial relations, and then crossbred the dogs. They found that the first generation offspring tended to have intermediate performance on behavioral tests relative to their parents, although behavioral patterns grew more complicated in subsequent generations. Nearly three decades later, geneticist Jasper Rine performed a crossbreeding experiment of his own, breeding his Border Collie Gregor (named, of course, for Gregor Mendel) with his Newfoundland Pepper. The first generation of puppies, as with Scott and Fuller's Cocker-Basenji mixes, exhibited a combination of their parents' traits: they loved water like Gregor and hunted down tennis balls with the eagerness of Pepper. But Pepper and Gregor's traits weren't so clearly combined in the next generation of pups, their grandpuppies. These puppies had some of Pepper and Gregor's traits, but the combinations of those traits varied from puppy to puppy. While the behavioral characteristics of Pepper and Gregor's children were predictable to Rine, the characteristics of their grandchildren were not. Just because one of a particular puppy's grandparents was a Newfoundland, that didn't mean the puppy had any interest in swimming.

There's another hazard to assuming a mutt's personality based on its presumed parentage: you might have the parentage wrong. When Scott and Fuller bred their Cocker-Basenji mixes, the second generation of pups showed a great variety in physical appearance and didn't look much at all like the original parents. (Edit: A study released by the National Canine Research Council found that dogs of unknown origin are frequently mis-labeled when it comes to breed.) Appearance, it turns out, isn't a reliable indicator of breed, so if you're using a dog's physical form as clues to its personality, you'll have little luck if that dog is a mystery breed.