among New World monkeys (10–14). Although there is some limited variation in the set of M/L photopigments for different species, in each case the sorting of these pigments among individuals appears to be the same as we found for the squirrel monkey. Because there are still many species that have not been investigated, we do not yet know if this pattern is universal for New World monkeys.
A second surprise about New World monkeys was that their color vision variations have a singular sex-linked component. Although individual female monkeys can have either dichromatic or trichromatic color vision, all the males are dichromats (15,16). The genes that specify the opsins required to produce M and L cone photopigments are on the X chromosome. This fact suggested a simple model to explain the polymorphism of cone pigments and color vision in these New World monkeys (7,17). The idea is that there is a single locus on the X chromosome of these monkeys with three allelic versions of the opsin gene. Each gene specifies one of the three possible M/L pigments. Male monkeys have one of these three genes; in combination with the S-cone pigment [the opsin of which derives from a gene on chromosome 7 (18)] males thus get one of three types of dichromatic color vision. Homozygous female monkeys will also have dichromatic color vision, but heterozygous females inherit genes for two spectrally distinct M/L cone pigments. The mechanism of X-chromosome inactivation sorts these two into separate cone classes and trichromatic color vision emerges. Studies employing both classical pedigree analysis (15) and molecular genetic approaches (16,19) have provided strong support for this model.
Old World Monkeys and Apes. Color vision in Old World monkeys and apes presents a quite different picture. As far as we know, all the species from these two groups have trichromatic color vision (20). Direct measurements of the M/L photopigments in these species are rather sparse, but it appears that the λMAX values for two types of pigment are at about 530 and 560 nm, respectively (21–23). The opsins for these pigments arise from the activity of two different types of gene on the X chromosome (24–26). There is so far a remarkable absence of any evidence for polymorphism of these photopigments and consequent individual variations in color vision in any of the Old World monkeys or apes.
Classical Variations in Human Cone Pigments. Polymorphic variations in M/L cone photopigments are common among people (affecting a total of about 4% of the population). These lead to the color vision defects and anomalies that have been the subject of intensive study for many years (27). As estimated from a variety of different experimental approaches, the M/L cone pigments of normal human trichromats have spectral peaks of about 530 and 560 nm (28–31). Absence of either of these types leads to dichromatic color vision —deuteranopia and protanopia, respectively. A second major class of polymorphic variation in the M/L pigments produces the most common color vision defects, the anomalous trichromacies. In this case, the standard explanation has been that either the normal M or the normal L pigment is replaced by an “anomalous” pigment, and this anomalous pigment is peak-shifted so as to be very close in spectral position to that of the remaining normal pigment (32). The reduced spectral separation of the two pigments, perhaps in combination with other factors, accounts for the aberrant color discrimination that is characteristic of these individuals. The actual spectral positions of these anomalous pigments are not as securely established, but by many accounts the peak separation between these pigments and the remaining normal pigments may be about 6 nm (e.g., see ref.33).
A compilation of the measurements of M/L cone pigments in nonhuman and human primates suggests that all primate color vision in this part of the spectrum is subserved by a restricted set of available pigment types. There may be only six of these. Fig. 1 shows the absorption spectra for these pigments, and it is noteworthy that the same pigment positions are represented in many different species of primate. For instance, every Old World monkey, the apes, people, and (with one known exception) all New World monkeys share in common a version of the M/L pigment that has a spectral peak at about 560 nm. The mechanism controlling spectral positioning of primate pigments must be conservative.
Additional M/L Pigment Polymorphism in Humans. Evidence has accumulated over the past decade to indicate that there are measurable variations in the spectra of human M and L pigments beyond those that produce the classical color vision defects. It has long been apparent that human subjects of the same color vision phenotype often make reliably different color matches. In a thorough series of psychophysical experiments, Alpern and his colleagues documented these individual variations in color matching, and in so doing convincingly demonstrated that they must be attributed to individual variations in the spectral positioning of the M and L cone pigments (reviewed in ref.34). Although most psychophysical experiments have confirmed the presence of variation in the spectral positioning of the human M/L pigments, both the extent and nature of the variation have been subjects of spirited debate (for a recent review of this work see ref.35). On the one side are experiments involving Rayleigh matching in which the distribution of matches made by trichromatic subjects is multimodal (36–38); in other experiments, however, the match distribution is found not to be multimodal (39,40). The interpretational difference between these two sets of experiments is whether or not the match variations of normal human subjects can be considered to reflect an additional polymorphism of the human M and L photopigments. That these behavioral experiments have not yielded a common outcome could well reflect the small size of the variations that are being measured and the inevitable differences arising from variations in experimental techniques. Although the behavioral experiments are ambiguous on the possibility of additional polymorphism of human M/L pigments, recent work on human cone opsin genes and their pigment products shows in convincing fashion that such pigment polymorphism does exist.