I shouldn't need to ask this, as I am almost certain I know the answer, but I will do anyway

Is it possible for two Optigen tested PRA-clear dogs to produce PRA affected/carrier puppies ?? I thought it was a simple recessive inherited disease, but comments made on another forum suggests that it is possible for affected puppies to be born to clear-tested parents, and that research is ongoing.
I can't find anything on the Optigen website about this - does anyone know if anything has been published, yet??