Megacolon is more common in dominant blaze and dominant white spotting (DWS). Recessive blaze and variegation on the h locus is not linked to this, though any rat has the potential to carry megacolon. There are people working on breeding megacolon out of their DWS lines. If you work with DWS you must be prepared to cull any rat that shows sign of megacolon, and to watch out for late onset megacolon.