Figure 1. Tuning of cone photopigment absorption spectra. (a) Absorption curves for S (blue curve), M-class (family of green curves) and L-class (family of red curves) pigments. Wavelength of peak absorption is 415 nm for S pigment, near 530 nm for M-class pigments and near 560 nm for L-class pigments. The rectangular bar below the x axis indicates the color appearance of different wavelengths to a person with normal color vision. (b) Two-dimensional representation of L and M opsins. Balls represent amino acids. Gray balls are invariant amino acid positions among normal L and M opsins. The black ball is the residue to which the chromophore is attached. Red balls are the two amino acid positions that produce the spectral difference between M- and L-class pigments. Yellow balls are positions that produce small spectral shifts and produce subtypes of M and L pigment. Blue balls are variant positions with no influence on the spectrum.
Figure 2. Comparison of dichromatic and normal color vision. (a) The colors of the visible spectrum as they appear to a person with normal color vision (left) were digitally altered (right) to illustrate the appearance of the same spectrum to a red–green color blind dichromat. (b) Photograph of red and green peppers (left) digitally altered to illustrate the appearance of the same peppers to a red–green color blind dichromat. There are two properties of color: hue and brightness. A person with normal color vision can detect the difference in hue between bell peppers that do not differ significantly in brightness. A dichromat cannot detect the difference in hue, and the peppers appear to be all the same color.
Figure 3. Recombination between X-chromosome pigment gene arrays required to produce arrays observed in the present-day population underlying normal, protan and deutan color vision. (a) Intergenic recombination between ancestral two-gene arrays that confer normal color vision gives rise to one new array that confers normal color vision and another that confers dichromacy (deuteranopia). (b) Intragenic recombination between two two-gene arrays that confer normal color vision produces two new arrays that both confer color blindness. (c) Intragenic crossover needed to produce protanomalous arrays. The parental three-gene array must be produced by crossover between two ancestral two-gene arrays, and this added step probably accounts for the lower frequency of protanomaly in the population. (d) To delete the M gene from a deutan array requires a crossover between a deutan array with an M gene and another array.