Division of Medical Genetics and Department of Genome Sciences, School of Medicine, University of Washington, Seattle, WA, USA
Humans and Old World (OW) primates have rich trichromatic color vision because they have three classes of retinal cone photoreceptors: short-wave-sensitive (SWS, blue), middle-wave-sensitive (MWS, green), and long-wave-sensitive (LWS, red).1 Like most mammalian species, most New World primates have dichromatic color vision that is based only on SWS and MWS classes of cones. Trichromacy in OW primates was acquired due to duplication of the X-chromosome linked gene encoding the MWS photopigment, followed by divergence to form X-linked MWS and LWS pigment genes. This happened about 40 million years ago after separation of the NW from the OW lineages. Interestingly, an independent MWS gene duplication occurred in the howler monkeys (Allouatta spp.) that similarly gave rise to trichromatic color vision in this species.3 Trichromacy also exists among females of other species of NW monkeys.4 This is because their single pigment gene on the X-chromosome is highly polymorphic in relation to the spectral sensitivity of the encoded pigment. Females who are heterozygous for two spectrally different forms of the pigment have trichromatic color vision. X-chromosome inactivation in such females ensures expression of only one pigment per photoreceptor cell, which is critical for color vision. There is good evidence that evolution of trichromatic vision among primates involved selection for the ability to distinguish between young and adult leaves and between ripe and unripe fruits.2
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3. Jacobs GH, Neitz M, Deegan JF, Neitz J. Trichromatic colour vision in New World monkeys. Nature. 1996;382:156–158.
4. Rowe MP, Jacobs GH. Cone pigment polymorphism in New World monkeys: are all pigments created equal? Vis Neurosci. 2004;21:217–222.