Dichromat
| Dichromacy | |
|---|---|
| Classification and external resources | |
| Specialty | ophthalmology |
| ICD-10 | H53.5 |
| ICD-9-CM | 368.5 |
| OMIM | 303900 303800 190900 |
| DiseasesDB | 30022 |
| MeSH | D003117 |
Dichromacy (di meaning "two" and chroma meaning "color") is the state of having two types of functioning color receptors, called cone cells, in the eyes. 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, trichromats require three pure spectral lights to match all colors that they can perceive, and tetrachromats require four.
Dichromacy in humans is a 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.
There are various kinds of color blindness:
The three determining elements of a dichromatic opponent-colour space are the missing colour, the null-luminance plane, and the null-chrominance plane. The description of the phenomena itself does not indicate the colour that is impaired to the dichromat, however, it does provides enough information to identify the fundamental colour space, the colours that are seen by the dichromat. This is based on testing both the null-chrominance plane and null-luminance plane which intersect on the missing colour. The cones excited to a corresponding colour in the colour space are visible to the dichromat and those that are not excited are the missing colours.
It is more informative to use situations where less than the total visual system is operating when studying about vision. For example, a system by which cones are the sole visual receptors could be used. This is rare in humans but certain animals possess this trait and this proves useful in understanding the concept of dichromacy.
While their Triassic ancestors were trichromatic,placental mammals are as a rule dichromatic; the ability to see long wavelengths (and thus separate green and red) was lost in the ancestor of placental mammals, though it is believed to have been retained in marsupials, where trichromatic vision is widespread. Recent genetic and behavioral evidence suggests the South American marsupial Didelphis albiventris is dichromatic, with only two classes of cone opsins having been found within the genus Didelphis. Dichromatic vision may improve an animal's ability to distinguish colours in dim light; the typically nocturnal nature of mammals, therefore, may have led to the evolution of dichromacy as the basal mode of vision in placental animals.
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