but when you get the feeder breeders buying these "pretty" rats instead of the safe patterned/colored rats, you get megacolon babies and carriers.
So you're going to sit there and say that anyone who breeds a high white is guaranteed to get megacolon babies and carriers simply because they're feeder breeders? Believe it or not, but it's not a biased gene that automatically appears simply because people aren't being "responsible" breeders. People who're breeding for fancy feeder breeders can in fact luck out and produce healthy lines that aren't carrying megacolon. Actually, some of the feeder breeder rats are healthier than the "common" or more readily available pet rat lines, simply because they cull and cull hard for unhealthy rats. Nobody breeding solely for feeders is going to hold onto rats that die at 8 weeks from megacolon (well, nobody is), and most will ditch the parents that produce weak offspring, simply because it's not good for production.