For female rats you want to wait until they're 5-6 months old and/or 250 grams. It'll insure that the female can handle the pregnancy and give you a good liter size. My latest first-time mother just gave me 12 babies. For males age doesn't matter as much, but usually the girls won't mate unless the male is bigger. Males usually grow faster than females and I haven't had any trouble breeding two rats of the same age, so long as I give the female enough time to mature.

I'm pretty new with their genetics myself, so all I can really say about patterns is that it depends on the amount of white on the rat. If you breed two with similar amounts of white and/or similar markings you should produce similar looking babies. All my males have been nearly one color with just a bit of white on them and I've noticed that when bred with different females the amount of white changes. Females with more white produce babies that have more white than the father. Females with less white produce babies with little to no white. I'm trying to breed the white out of some of my colors so I can produce selfs (single color rats) so this result is ideal for me.

What fascinates me about rats is that a lot of the colors aren't compatible. My first pairing was a brown (lilac) female to a gray (sky blue?) male and all of the babies were black. I kept a male and a female from that litter and now that they're grown up I'm hoping that I'll be able to produce the colors of the parents by breeding the siblings together. The female had her first litter a week ago and their colors are just starting to come in, so right now I can tell that some are going to be lighter than others, but I'm still not sure I'll get the exact colors I'm looking for.