My answer to this one is mega colon also. If I remember right, mega colon shows up in "blaze" genes quite often. But I cant remember the specifics on if it is paired with another gene or what. I never worked with the "blazes" so I am not totally sure much else. But I am sure you will find it if you look around the net. If you look at that female, she has the charicteristic "blaze" mark, that white on her forehead. Maybe the father who carries this gene has a "blaze" and the other dont? Hmm, hopefully you can identify which male it is. Any babies made by the offending father/mother will carry that gene, and could produce babies of thier own who have problems with mega colon. They are always carriers for that gene, but some never are effected by it physically. Eliminate the rats that are the gene carriers from your breeding and you will do fine.