He is either not carrying the gene for RP or the RP gene has mutated.
Explanation:
RP or Retina Pigmentosa is an eye defect inherited by a person from a parent. The disease is degenerative as the individual gradually loses his sight. It normally starts off as night-blindness and could go as bad as a total loss of sight.
The mutation of both the RPGR and RP2 genes which are responsible for the function and structure of light-reception in the eye accounts for most cases of Retina Pigmentosa disease.
