If the original individuals of a line of dogs, or any other group of organisms, had certain genetic defects, then those defects would become concentrated in their descendants. For example, if I had a population of 1000 people starting a colony in another solar system and 500 of them had diabetes and the other 500 had the genes for several types of cancer, then their chance for survival would be bleak indeed. If however, the original founders of the line in question were picked carefully, then the line would not degenerate. My favorite example of this in Humans is a very small tribe living in the Amazon basin. They only number several hundred strong and are extremely isolated from other tribes. this isolation has been in place for thousands of years and yet the rate of birth defects in their population is only around 5% that of the general population. The leading cause of this genetic fortitude is do in large part to their practice of smashing a large stone into the head of any child born with a birth defect or that's born sickly. While this may sound cruel, modern medical technology and abortion procedures are unknown to them, it is very humane in the long run as the percentage of those who would be born with and have to suffer with birth defects, have been reduced to near zero. I would say that this group is about as close to a pure breed of Human as on can get.

Lee, (it's not the size that matters, but the quality)