Position 180 (in exon 3) of the L-cone opsin gene codes for the amino acid serine in 56% of the population but for alanine in the rest.
(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 x-axis corresponds to L-cone excitation and is also the fraction that the L-cones contribute to luminance, with:
(69) Therefore hybrid genes which derive their fifth exon from the L-cone opsin gene encode pigments whose peak sensitivity values cluster around the L peak sensitivity (within about 12nm), whereas those in which the fifth exon comes from the M-cone opsin gene cluster around the M peak sensitivity (within about 8nm).
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.
It is not yet clear whether the photopigment optical densities of the M- and L-cones are the same.
(40,41) The relative numbers of L- and M-cones vary greatly in the population, but on average, there are twice as many L-cones as there are M-cones in the central retina.