Megacolon is not that easy to predict or breed out. Neither does it show up at weaning, with some cases appearing 6-8 months down the road. If it was, don't you think it would be a non-issue?

That's why it is heavily advised to stay away from high white rats, because a rat can carry it and not be affected. However, it does not only affect high white rats, but most often megacolon is found in rats that also have high white spotting or lethal spotting.

If you would like to read more on megacolon, refer to ratbehavior.org or AFRMA.org.