First came working dogs — breeds that could guard or herd livestock. Later came the hunting dogs. And all those cute little fluffballs like Pomeranians and Papillons? Thank a single import of a pug from China for donating its genes for small size. Researchers at the National Institutes of Health spent more than 20 years sampling the genes of 161 breeds of dog, sequencing them and comparing them, and have come up with the most complete and definitive canine family tree. A toy Xoloitzcuintel, a breed that likely descended from dogs that crossed the Bering land bridge with Native American ancestors. Penny Inan / Cell Press It shows how breeds were mixed and matched to make new breeds, and gives a rough timeline and geographic map of what came from where. Even though the species Canis familiaris includes bassets and Samoyeds, poodles and Weimaraners, they’re all extremely similar genetically, says Elaine Ostrander of the National Human Genome Research Institute, part of the NIH. “All these breeds are members of the same species. They all have their chromosomes organized the same way — same genes, same order,” Ostrander said. “It’s very subtle variation in small numbers of genes that account for that very large difference in morphology that we see across breeds.” So while it might take hundreds of mutations to mark the difference between a human who’s 5’6” and one who’s 6’6” tall, Ostrander said, it only takes a “couple of dozen” genetic mutations to make a Chihuahua different from a Great Dane. The goal is to track disease-causing genetic mutations, which often translate to human disease genes, Ostrander says. “We have a yellow brick road for figuring out how mutations move around the dog world,” Ostrander told NBC News. “We recognize that everything humans get, dogs get — epilepsy, cancer, diabetes, heart disease,” she added. But in dogs, these disease traits often pop up more commonly in specific breeds. “We can actually trace diseases as they move around the dog breed population,” Ostrander said. “For instance, collie eye anomaly is a disease that affects … several herding breeds, including the collie, Border collie, Shetland sheepdog, and Australian shepherd,”