I guess it's been about 4-5 years now for breeding both rats and mice. I started off with 2.6 black hooded rats, they had quite a few interesting genes though so now I'm producing Blacks, Whites, Blues, Browns, Champaignes, in all different shades either solid colored or hooded and many with dumbo ears.

With the mice I started out with about 20 white females and about a half a dozen white males, then a few months later added a couple of brown females. For a few years after that, all of my holdbacks for future breeders were all white females and all brown males. This way I could always tell which were the females and which were the males at a glance.