Content Summary : Parker et al. looked at microsatellites to find the genetic differences between breeds of domestic dogs. Microsatellites are short segments of DNA that contain repeats of DNA sequence. Therepeats usually occur in a noncoding part of the gene, and their number is highly variable. Analysisof the microsatellites of 414 dogs representing 85 different breeds revealed that the degree ofgenetic differentiation between dog breeds is much higher than that found between humanpopulations on different continents! Given that most modern dog breeds have existed for fewer than 400 years, it is surprising that dog breeds are genetically distinct. But a dog can be matched to its breed by its individualgenotype. Of 414 dogs tested, only 4 dogs were assigned to the wrong breed. This apparent genetic isolation of dog breeds through selective breeding was reinforced by the formation of breed clubs in the mid-19th century. Rules such as the "Breed Barrier Rule" states that"no dog may become a registered member of a breed unless its dam and sire are registeredmembers". Such selective breeding generates not only genetically diverse breeds of dog but alsoleads to the accumulation of mutations and inherited diseases. By using a genetic classification ofdog breeds, scientists will be able to select breeds of dogs that share the same ancient mutationsand genetic predisposition to diseases that some humans have. Analysis of this DNA is more likelyto yield information about the diseases and the mutations responsible for them.