Red-Light Thresholds in Heterozygote Carriers of Protanopia: Genetic Implications.

Absolute thresholds in response to red light were compared in nine normal subjects, six female carriers of protanopia (heterozygotes), and six male subjects with protanopia. The fovea and four peripheral retinal areas were tested, and all data were obtained before the occurrence of the rod-cone break. Elevated thresholds were found in all retinal areas tested in protanopic males, at the fovea in all carriers, and in some peripheral retinal areas in two carriers. The thresholds for carriers were far below those for the protanopic males, and no greater variability of threshold was found in the carriers when they were compared with the normal control group. The findings do not substantiate the occurrence of inactivation at the locus on the X chromosome for protanopia.