Inbreeding With Rats?

[Bogertophis]

...I have not brought in new blood in my colony in 9 years (my colony was started 12 years ago) and my animals are perfectly healthy (well until the inevitable)

I brought in new blood in my mice colony because I was tired of dark mice and it was the worse mistake I have made, and now my colony is not what it used to be production wise and I am considering starting back from scratch.

Key word is SELECTIVE breeding, hold back strong healthy genes and you will produce just that for how long? I do not have that answer, again so far 9 years and going strong...

I got like 2 new mice about 10 years ago, but otherwise, my mouse colony has been going for at least 25 years now with no problems. Granted, I had a many more
initially than I do now & I have always had an enormous variety. My system to avoid inbreeding is simple: I have all different "fancy colors" & I continuously mix them
up...a solid black mouse is paired with gray & white, the gold & white are paired with chocolate, etc. When I used to sell mice I also had "silky" & "satin" mice, but
they don't reproduce well so once I no longer sold mice to others, I was happy to quit those. In all these years, my mice are healthy & produce big litters...but it comes
down to personally taking the place of "nature" as to which ones are breeders & which ones have the "short straw" as feeders. Like Deborah said, you hold back & breed
the best ones, & it's a judgement call that gets more accurate with practice. I rarely have or keep any albino mice either...a few show up, but most don't "make the cut".
It's like the old saying, "the wolf keeps the caribou strong" [by eliminating the old, weak or sick ones]...only it's me along with my snakes that function as "wolves".