Not to depress folks, but even well documented lines can have "glitches" from time to time and produce hitherto unknown genetic problems.
Yep, I wanted to expand on this a little.. a lot of good breeders have already posted on this thread, but I just wanted to make sure that everyone understand that these things happen to *everyone*. No matter how much planning, how much research, and how much heart goes into a breeding, nothing is ever certain. ALL breeders will inevitably have to deal with the heartache of litters that just don't work out as planned, or brought together recessives that were unknown from either line.
I've had it happen myself.. I got two babies with hydrocephalus out of two completely unrelated rats, and I was devastated. That was and has been one of the worse experiences of my life.. I felt like I had failed those rats.. rats that barely had a chance in this world, rats that *I* was responsible for creating. However, no amount of researching prior to breeding that litter would have shown me any reason not to do it.. the information I found out after the litter, was only found out because I was asking about a fairly specific and not very common condition... the connections never would have been made if I didn't know what to look for.
Sometimes rats in lines without any known birthing issues will suddenly have some serious problems. I had that happen with two girls of mine (sisters), they each lost more than 2/3 of their litters due to what I beleive was placental abruptions. They lost a lot of blood in the deliveries, with blood gushing out between each baby, and I honestly think it's a miracle that both moms survived. That was *very* hard to handle. I don't honestly know if I will continue that line or not.. it was just horrible to witness, and I don't want to be responsible for it happening again. However other related rats (siblings) of those two girls have had normal healthy litters, so it's possible it was just a bad genetic mix between those girls and the boy I bred both of them to.. I just don't know, and it's hard to make determinations without a lot of information.
I also had a mother last month *kill* an entire litter of 3 week old babies when I went out of town.. I will never know for sure what happened, but something snapped in her head and then they were all gone, and that was a *horrible* thing to come home to. Obviously she's retired now, and I was already planning on not breeding from that line anymore in general to focus on other things, but that doesn't take away any of the pain.
Breeding is not all about "cute babies".. there's a lot of hard decisions and broken hearts involved, and sometimes it can be VERY hard to handle it. That's part of the "new breeder" phenomenon... someone wants to breed rats because they love them, they come across their first issue or bad litter(s) and they up and quit. Very few breeders that do that accomplish anything more than just making more rats.. no matter where their "foundation" rats came from. The breeders that stick with it, change the way they do things, stop certain lines, etc, are the ones who are actually accomplishing something positive... not the ones who throw a few rats together to get some "great pets". In breeding ANY kind of companion animal that is a given, but WHAT else are they trying to accomplish?
The best compliments I've ever gotten from breeding are hearing back from people who have adopted rats from me and write me months or even years after getting them, and they go on and on about how wonderful those rats are.. from breeders telling me that my rats are some of the best they've ever gotten.. it's those encouraging words that get breeders through the "hard times"..