32) The gene that codes for the S-cone pigment is found on chromosome 7 and those encoding the L- and M-cone pigments are located on the X-chromosome.
The S-cone pigment gene has 348 codons and shows a 46 [+ or -] 1% similarity to either the L- or M-cone pigment gene.
Firstly, the L- and M-cone opsin genes are polymorphic.
32,37) Most studies indicate that one L-cone opsin gene is found at the beginning of the array, and is followed by one or more M-cone or hybrid opsin genes.
The L- and M-cone sensitivities used to construct this diagram summate to the photopic luminosity function, whereas the S-cone sensitivities are scaled in any convenient way, although most frequently so that the maximum b value is 1.
Since the spectral luminosity function is the sum of the L- and M-cone spectral sensitivities, a protanope's spectral luminosity function peaks at a lower wavelength than that of a normal trichromat and shows reduced sensitivity to long wavelength light.
The main cause of protanopia and deuteranopia lies in the unequal intergenic and intragenic recombination of the L- and M-cone pigment genes.
As already mentioned, only 15 amino acids distinguish the L- and M-cone opsins, but the greatest cumulative shift in spectral sensitivity (about 25nm) can be traced to the differences in amino acids encoded for in exon 5.
The existence of L-cone and M-cone monochromacy has been reported, but these are believed to have at least a partial post-receptoral origin.
18-20,23,24) In the central 2[degrees] of vision, the S-cone photopigment density is thought to be 5-20% lower than that of the L- and M-cones.
The L- and M-cones are randomly distributed across the retina and greatly outnumber the S-cones.