i sometimes keep 1 male of a litter.then save 2-3 females. i just mix them up. i know a lot of inbreeding happens there but so far, out of alot of rats, only 2 have shown deformity. their heads started to tild sideways. and when the girl had a litter, only 1 survived and the pup's eyes was closed even after 4 weeks. so i just fed them off. the large rats to a burmese and the pup to a ball.