Elaine A. Ostrander in American Scientist:
A pekingese weighs only a couple of pounds; a St. Bernard can weigh over 180. Both dogs, though vastly different in appearance, are members of the same genus and species, Canis familiaris. How dog breeds can exhibit such an enormous level of variation between breeds, and yet show strong conformity within a breed, is a question of interest to breeders and everyday dog lovers alike. In the past few years, it has also become a compelling question for mammalian geneticists.
The “dog genome project” was launched in the early 1990s, motivated by scientists’ desire to find the genes that contributed to many of the ills suffered by purebred dogs. Most dog breeds have only been in existence for a few hundred years. Many exhibit limited genetic diversity, as dog breeds are typically descended from a small number of founders, created by crossing closely related individuals. Further, breeds often experience population bottlenecks as the popularity of the breed waxes and wanes. As a result of this population structure, genetic diseases are more common in purebred dogs than in mixed-breed dogs. Scientists have been motivated to use dog populations to find genes for diseases that affect both humans and dogs, including cancer, deafness, epilepsy, diabetes, cataracts and heart disease. In doing so we can simultaneously help man and man’s best friend.
More here.