What is the chances in rats for megacolon?

What are the chances of megacolon of the mommy and litter are NON high white. Mom was dumbo Mink Berkshire, dad was Dumbo Russian Blue Self. The babies range from Mink, Mink Dilute, Black, and Beige. The markings are Irish and Irish mismarked. The babies are 4 weeks and seem to have a bit of belly pudge. Is it normal for babies this age to have a slightly chubby belly? They all seem to be going to the bathroom fine...quite often actually.