I definitely would NOT breed her, especially if you don't know her genetics(pet store rat). The breeders I know have pedigrees on their rats for 5-6 generations, and if there is a problem in any generation, the whole line is removed from breeding.

There are enough megacolon carrying rats, and the professional breeders try incredibly hard to get rid of it out of their colony.

I have a Black-Eyed White top eared male from a bad line and his health is horrible. There wasn't megacolon in the line(as it had never occurred in the 7-8 generation family tree) but his mother and father were both aggressive and not as healthy as they could have been. I personally wouldn't have bred them, but it was the breeder's choice... Moose(RMAX The Moose Is Loose), has chronic respiratory issues and lung scarring. We have to keep a humidifier going in the room so he doesn't get too dry and wheeze.

But anyway, like I said. If you don't know her background in the least little bit, then it would be wise to not breed her.