Serine/alanine amino acid polymorphism of the L-cone photopigment assessed by dual Rayleigh-type color matches.
The dual Rayleigh-type color match is the ratio of 621 nm light to 550 nm light that in admixture matches 586 nm light, divided by the ratio of 667 nm light to 550 nm light that in admixture matches 586 nm light. Compared to the classical Rayleigh match, the dual-match procedure minimizes variation in color matching arising from differences in lens pigmentation and photopigment optical density, and thus amplifies individual differences due to shifts in L pigment lambda max. We hypothesized that the dual matches would provide a clearer distinction between subjects with serine and subjects with alanine than would the classical Rayleigh match because individuals with serine express L pigments with a lambda max shifted toward longer wavelengths than do those with alanine. Classical Rayleigh color matches were compared with dual Rayleigh-type color matches in 14 color-normal observers whose DNA had been analyzed previously for the presence of the amino acid serine or alanine at position 180 in the L opsin. The resulting distribution of dual-match measurements for the seven subjects with serine does not overlap the distribution of measurements for the seven subjects with alanine. The classical Rayleigh-match measurements for these two groups of subjects, on the other hand, overlap substantially. More than half of the subjects' classical Rayleigh matches are within the overlapping range. The dual Rayleigh-type matches, therefore, provide an improved psychophysical technique for assessing whether an individual observer has serine or alanine at position 180.