Quote Originally Posted by mykee View Post
Yes, inbreeding the same line of rats for too many generations (more than 10 or so) will definitely cause issues. If you do not introduce new blood (by way of new breeder males) into your collection every year or so, you WILL get babies that are very small, deformed, weak and females that will produce smaller litters or none at all.
This will only happen if you haphazardly breed animals. If you select your breeding stock carefully you can avoid these problems.