Just a few genes can make a huge difference
A comprehensive study co-led by researchers at Stanford, Cornell, and the National Human Genome Research Institute discovered that the majority of breed-defining characteristics in dogs such as body size, coat characteristics, and ear floppiness, are determined by a small number of genetic regions with large effect. In humans and other species studied thus far, most physical characteristics are influenced by many genetic variants, each of which has a small effect.
The researchers suggest that this dominance of a few genes of large effect resulted in part from the Victorian love of novelty. Many dog breeds were created during the Victorian Era, when breeders favored distinctive mutations such as the gene responsible for short legs found in corgis and other “dwarf” breeds. In contrast, other domestic species were bred for economically useful traits such as fast growth and fertility, which involve subtle differences among many individuals.
For more information see: http://www.sciencedaily.com/releases/2010/08/100810203503.htm
The original article can be found here: http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000451