Progressive cone or cone-rod dystrophies (CORDs) characterized by a defective cone function demonstrate abnormalities in cone-mediated electroretinogram (ERG) components. The presenting symptoms are defective color vision, impaired central visual acuity and sensitivity to light (Small and Gehrs, 1996; van Ghelue et al., 2000). The inheritance patterns for CORDs are autosomal dominant, autosomal recessive and X-linked (Michaelides et al., 2005a). The preservation of rod function in CORDs can differ both between and within families and depends on the disease causing mutation within a gene (Small and Gehrs, 1996; Michaelides et al., 2005b).