You want the females to be considered "medium" rats before you start breeding them.
Megacolon is, in short, a disease which affects the colon, causing it to enlarge. The long version is here: http://ratguide.com/health/digestive/megacolon.php
Hooded rats are the only ones (as I understand it anyway) that typically don't have genetic problems with megacolon (that's due to the genetic trait for megacolon being attached to the gene for pigment migration), and thus the reason why I am trying to hold back hooded rats to use as breeders. I only have one picky ball python as far as color goes, and even she will eat rats that are black and white (but she refuses solid black rats). That's not to say that any other color rat will absolutely have issues with megacolon, but hooded rats have the least chance. If I were breeding for certain colors/patterns I would try to determine which, if any, carried the megacolon gene and euthanize, but since I'm only breeding for feeders, color and pattern doesn't matter so much to me, so I go with the ones I hope will have lesser issues.![]()