Dichromat
A
dichromat is an organism that 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. The condition of being a dichromat is called
dichromacy.
The normal explanation of dichromacy is that the organism's
retina contains two types of
color receptors (called
cone cells in
vertebrates) with different
absorption spectra. In practice the number of such receptor types may be greater than two, since different types may be active at different light intensities (in other words, the absorption spectra of different types of color receptors may overlap). In vertebrates with two types of
cone cells, at low light intensities the
rod cells may contribute to color vision, giving a small region of
trichromacy in the color space.
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.
The two best-known forms of
color blindness in humans result in dichromacy, since one of the three cone systems is non-functional in these conditions. However, many people who are described as color blind are in fact anomalous trichromats; in this condition, there are three functional cone systems but one of them has an unusual absorption spectrum so the person does not make the same color matches as the rest of the population.
*
Visual comparisons of various types of color vision impairments by
Cal Henderson*
Color Vision, Color Deficiency at Firelily Designs
*
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.
