Not a lot is known about the genetics of white coloration in dogs, but most of the mutations seem to be recessive. If the dog looks like a Yorkshire Terrier, I doubt that the white color is due to an accidental breeding. Have the ancestors of this dog always bred true for normal Yorkie colors? The reason I'm asking is that an extreme parti-colored Yorkie could appear to be all white...
See
White Coat Color in Dogs for a review of what is known about the genetics of white coat color in dogs.