Just like with dogs, a rat brother and sister may not have genes that are alot alike. A cousin may be more closely related to a rat than it's own sibling. So really, it isn't like inbreeding human siblings whose genetic makeups are alot alike. Inbreeding or linebreeding is done to strengthen a certain characteristic. If you breed rats that aren't related, you really don't know what you are going to get in the way of personality and body structure -- you can only hope. If you breed rats that are related, you can pretty much be sure of what you are going to get. That's why you'll notice that alot of dog breeders have dogs that look alot the same -- they are linebred so that they know what the puppies' "type" will be, since most breeders strive for a certain type. However, if you find that there is alot of inbreeding in the line, I'd stay away from the breeder. Too much can cause alot of problems. I've seen rats in pet stores that are so severely inbred that they were born without eyes.