You've probably seen it on TV or on YouTube. Case One: A dog lies on the couch chewing a bone.His hind leg starts to twitch repetitively.He tenses up and stares back at it anxiously as though it’s an intruder sneaking up on his bone. His raises his lip, he growls. Then blam! He attacks his leg. Canned laughter. $100,000 prize winner, America’s Funniest Home Videos. Case Two: A Bull Terrier spins repetitively in the bathroom and then in the living room.

Dogs come in countless shapes and sizes and exhibit more diversity than any other land mammal on earth, a fact that makes them especially appealing to geneticists. In the last decade, scientists working at more than 100 laboratories worldwide have made significant progress in painting a detailed picture of the complex relationships between physical traits, behavior and disease in purebred dogs, and the ways genes contribute to the striking differences seen across breeds.