Maintaining and improving quality in an established and existing line is just as hard – if not harder – than increasing quality on a line from very poor origins. It takes very little real work to take a line from poor quality to acceptable quality (to a large degree, just feeding, raising, and housing them in better conditions for a few generations will make them much better, nevermind any selection you do). But moving a rat from being acceptable quality to very good quality is harder, and actually maintaining a top quality line is very difficult.
Rat breeding is about spinning lots of plates in the air at once, not about checking things off a list. You can’t breed to get one aspect right, tick it off as “done”, then move on to something else. You need to constantly be spinning each plate enough to ensure that it’s not going to fall down, carefully watching to ensure none of the other plates are wobbling too much.
It depends what you mean by your line. If you just mean rats you’ve bred then sure, the first time you mate a buck and a doe together and they successfully produce kittens, you have a line.
To me, someone has a line when they’re producing litters of rats that strongly resemble each other, that have a recognisable look and consistency to them. To do this involves a few generations of careful breeding – maybe you start with rats from one breeders line, maybe you are mixing rats from different backgrounds. But you’re selecting hard for something in particular, and you are producing results – then you have a line, before that you’re breeding rats that are from someone else’s lines.
Some people can breed for two or three generations and have a real line of rats. Some people can breed for many years and just have a hodge podge of rubbish that I wouldn’t consider a line.
Three or so generations, I reckon, of you breeding, picking, and keeping both sides of the pedigree, before you can think of it as yours. The first couple of generations are really just building on what you were given, and if you have decent stock you’d have to work hard to fuck it up that quickly. By the third generation, your selection pressures and husbandry are going to start having a decent amount of influence on the babies you produce.
I breed for show and pet in equal measure. I don’t think there’s any benefit to rats if we breed for anything less than the full package, and so I think every mating should be taking into account temperament, health, type, and the paint job (deliberately listed in that order).
It is hugely unusual to see anyone reputable offering kittens for homes that are only a couple of weeks old. Breeders should be interested in selecting which kittens they keep back for future breeding on the basis of best possible temperament, type, and colour/markings.
When you are looking at kittens that are very young, you know what variety they have and what markings, but you cannot properly evaluate temperament or type. So that means you are only selecting on what variety they are – not even the quality of it, since the colour and markings can change rapidly in the next few weeks.
If you do have an interest in breeding to improve temperament/type/colour, you should make decisions on the kittens you are keeping and those you are not as late as possible. This would allow you more time to properly evaluate the kittens, and it’s also better for those purchasing kittens, as you can tell them more about the personality of the rats involved rather than it just being about what they look like.
By properly selecting the rats you keep as potential breeders in the first place, you’ll be able to breed to improve, rather than breed just to create more kittens of a particular variety.