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Dichromacy is a moderately severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, affecting predominantly males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.

Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights. By comparison, a trichromat requires three pure spectral lights to match all colors in their visual spectrum.

Dichromacy in mammals

It is currently believed that most mammals are dichromats. The straightforward exceptions are Old World primates, including humans, which are usually trichromats, and sea mammals (both pinnipeds and cetaceans) which are monochromats. New World monkeys are a partial exception: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are monochromats, and both sexes of howler monkeys are trichromats. Recent research (e.g. Arrese et al, 2005) suggests that trichromacy may be widespread among marsupials.

External links

• Visual comparisons of various types of color vision impairments by Cal Henderson
• Color Vision, Color Deficiency at Firelily Designs

References

1. Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
2. "Guidelines: Color Blindness." Tiresias.org. Accessed September 29, 2006.

• Color blindness at Absolute Astronomy Reference
• Arrese, C. A., Oddy, A. Y., Runham, P. B., Hart, N. S., Shand, J., Hunt, D. M., * Beazley, L. D. (2005). Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (Setonix brachyurus) and quenda (Isoodon obesulus). Proceedings of the Royal Society of London Series B, 272, 791-796.
• Jacobs, G. H., & Deegan, J. F. (2001). Photopigments and colour vision in New World monkeys from the family Atelidae. Proceedings of the Royal Society of London, Series B, 268, 695-702.
• Jacobs, G. H., Deegan, J. F., Neitz, J., Crognale, M. A., & Neitz, (1993). Photopigments and colour vision in the nocturnal monkey, Aotus. Vision Research, 33, 1773-1783.
• Mollon, J. D., Bowmaker, J. K., & Jacobs, G. H. (1984). Variations of colour vision in a New World primate can be explained by polymorphism of retinal photopigments. Proceedings of the Royal Society of London, Series B, 222, 373-399.

• Vision