Trichromatic color vision with only two spectrally distinct photopigments

Protanomaly is a common, X-linked abnormality of color vision. Like people with normal color vision, protanomalous observers are trichromatic, but the ir ability to discriminate colors in the red-green part of the spectrum is reduced because the photopigments that mediate discrimination in this range are abnormally similar. Whereas normal subjects have pigments whose wavele ngths of peak sensitivity differ by about 30 nm, the peak wavelengths for p rotanomalous observers are thought to differ by only a few nanometers. We f ound, however, that although this difference occurred in some protanomalous subjects, others had pigments whose peak wavelengths were identical. Genet ic and psychophysical results from the latter class indicated that limited red-green discrimination can be achieved with pigments that have the same p eak wavelength sensitivity and that differ only in optical density. A singl e amino acid substitution was correlated with trichromacy in these subjects , suggesting that differences in pigment sequence may regulate the optical density of the cone.