Long before humans got too lazy to fetch their own slippers, they were shaping the appearance and behaviour of dogs to suit their taste and practical requirements. Until recently, assigning a dog to one of the 150-or-so breeds that we have today depended on having a trained eye or a catalogue of each breed's vital statistics. Now, Heidi Parker and colleagues have shown that genetic information can be just as reliable — not only for telling breeds apart, but also for describing their genetic history.

Dog breeding is serious business and the strict standards imposed by kennel clubs mean that the gene pools of different breeds never mix. This is a boon to geneticists, who can use genetic markers to define the structure of dog populations and, from a medical perspective, to find the genes that underlie canine models of human disease.

Parker and colleagues concentrated on 85 domestic dog breeds, and genotyped 414 dogs at 96 microsatellite loci — these being the most useful genetic markers for characterizing recent populations. The history of genetic isolation among breeds showed up clearly in the analysis: dogs within breeds were genetically much more similar than dogs from different breeds. Indeed, 30% of total variation is accounted for by among-breed differences, which is among the highest of all domestic livestock. This strong genetic differentiation allowed breed membership to be predicted from the dog genotype: with the exception of four breeds, dogs were consistently assigned to the same breed when they were clustered on the basis of similar microsatellite genotype (using the Structure statistical program). Microsatellite data also assigned each dog to its own breed in 99% of cases.

Having established the genetic pattern of modern breeds, the authors turned to some phylogenetic history. Using the wolf to root the tree, a combination of the neighbour-joining method and the Structure algorithm was used to reconstruct the genetic relationship among breeds. Four main clusters appeared: an ancient one corresponding to a mixture of African and Asian breeds, and three more modern clusters of European origin that each contained breeds of similar morphology and behaviour. As well as confirming genetically what our eyes and historical records had already suspected, this study also reminds us to be wary of judging by appearances. For example, modern-day Norwegian Elkhounds, which were thought to be direct descendents of Arctic breeds that existed 5,000 years ago, have in fact been recreated more recently from more modern breeds.

Dog breeding is an ongoing evolutionary experiment that we can use to our advantage, and knowing the dog's population-genetic structure makes it simpler to map important dog genes — whether it be those that underlie the 350 inherited models of human disorders or those that make the Pekingese 50 times lighter than the Irish Wolfhound.