When Bochem and Optigen were looking for the rogue gene that caused PRA in Glen of Imaal Terriers, and finding again and again it wasn’t what they hoped, some people mused on a possible modifier. This was due to the diagnosed PRA affected dogs that, in some cases, just didn’t “Pee” i.e. didn’t Progress as per the name. Other folk were very sceptical about that idea and it has now been proven (thankfully) they were right and modifiers weren’t involved. This link is for those who said but it is very, very rare for such things to crop up-as you can see the ones that were wondering weren’t totally tilting at windmills.