Just like Daniel described with his mice, that's what I do with my rats. I'm just starting on my next generation of breeders so I'm picking daughters of my best breeders. Also I'm looking at attractive colors or patterns I like (heck why not have cute rats if you have rats right LOL) as well as young females that have good sturdy body structures and decent temperments. My new breeder male is a recent outside purchase as I found an American Blue male plus I wanted some new blood in the colony anyways.