In dog breeding, breeders use the inbreeding coefficient to determine whether or not breedings are too close. When we inbreed or 'linebreed', eventually the puppies are physically weaker, smaller, less vigorous. They don't have as strong of a reflex to nurse, they aren't as fast, etc. So you use inbreeding coefficients to avoid the super tight breeding. Another symptom of closely bred populations is decreasing litter sizes.

In basenjis, we can go to africa and bring back native dogs to increase the genetic diversity of the breed. The native dogs are unfailingly more instinctive and hearty babies, hearty nursers, etc. The mother's have stronger mothering instincts than tight linebred mothers. The native dogs also produce giant litters. American stock basenjis produce an average litter of four or five. When native dogs are bred in or used exclusively as native stock the litters are averaging much bigger-- 8, 9, even 10 puppies.

I would assume that snakes have similar genetic issues. Inbreeding anything too closely for too long will decrease the heartiness of the babies and decrease litter sizes, etc, etc, etc. Good reason to use wild snakes in the gene pool!