The Genetics of Color Vision and Congenital Color Deficiencies

Primates are unique among mammals in possessing trichromacy. In Old World primates, it is based on three cone classes in the retina, each expressing a different class of visual pigment. These pigment classes are each orthologues of pigments present throughout the vertebrate kingdom, the short wavelength-sensitive (SWS1, SWS2, LWS and MWS) pigment and two representatives of the long wavelength-sensitive (LWS) pigment, L cone opsin and M cone opsin. 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 is 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.