Dichromacy

Dichromacy in humans 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, predominantly affecting 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.

There are various kinds of color blindness. Protanopia is a severe form of red-green color-blindness, where there is impairment in perception of very long wavelengths, such as reds. To these sufferers, reds are perceived as beige and greens tend to look like reds. Protanomaly is a less severe version. Deuteranopia consists of an impairment in perceiving medium wavelengths, such as greens. Deuteranomaly is a less severe form of deuteranopia. Those living with deuteranomaly cannot see reds and greens like normal people, however they can still distinguish them in most cases. A more rare form of color blindness is tritanopia, where there exists an inability to perceive short wavelengths, such as blues. Sufferers have trouble distinguishing between yellow and blue and tend to mistake greens for blues and yellows for reds.

Dichromacy in mammals
It is currently believed that most mammals are dichromats. The straightforward exceptions are primates closely related to 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.