Primates are unique among mammals in possessing trichromatic color vision. In Old World primates, it is based on three cone classes in the retina, each expressing a spectrally distinct class of visual pigment. These pigment classes are each orthologues of pigments present throughout the vertebrate kingdom, the short wavelength-sensitive (SWS1) pigment and two representatives of the long wavelength-sensitive (LWS) pigment, L cone opsin and M cone opsin. In primates, the latter two pigments arose from a duplication of the LWS gene that occurred at the base of the Old World primate lineage to give an array of two closely adjacent opsin genes on the X chromosome. This close proximity and the extensive sequence identity of the L and M genes promotes mispairing of the genes and thereby underlies the high frequency of red-green color blindness seen in humans. The consequences of this mispairing are the loss of either the L or M gene to give full dichromacy, or the generation of hybrid genes to give anomalous trichromacy. Generally, red-green color blindness is not associated with loss of acuity, although this is present in a rare form of dichromacy called Bornholm eye disease where cone dysfunction and myopia are also present. Other forms of color blindness include the X-linked disorder of blue cone monochromatism where L and M cones are absent, the dominant disorder of tritanopia where S cone are severely reduced or absent, and the recessive disorder of achromatopsia where all cone classes may be absent.