Trichromacy
Trichromats (Greek: tri-, " three times " and chroma, " color" ) are organisms which have three different types of cones as color receptors in the retina.
Occurrence
Among the higher mammals only the Old World monkeys are consistently trichromats. One of these is also the man. In some New World monkeys are about two-thirds of the females trichromats, creating a possible clue to the origin of the Trichromatie is given in primates.
Be found in bag mammals sometimes three different types of cones. These are likely to be a but obtained primarily Trichromatie. The other mammals, however, are dichromats. Birds, fish, reptiles and amphibians, however, are usually Tetrachromaten, which also trichromacy occurs.
Arthropods such as insects, crustaceans and arachnids are primarily trichromats, rarely occurs Tetrachromasie. In arthropods occurs next to the blue and green, a UV - receptor, while a red-sensitive sensory cell is absent in the trichromats among the arthropods.
Color vision
When there are three types of trichromates pins with different absorption maxima, the red-sensitive L- pin [Note 1], and green-sensitive M-cones [note 2] and the blue-sensitive S-cones [note 3]. From the measured values of the L-, M - and S-cones, the brain creates a color image.
The pins have a low sensitivity, so that they only work in good light conditions. At dusk or darkness whose functionality is greatly reduced. The sensitive rods take over the perception of light in the retina. However, since there is only one type of rods are and thus can not distinguish between different wavelengths, color vision decreases with decreasing brightness.
Formation in primates
The pigment for longer-wavelength light is encoded in all mammals on the X chromosome. Therefore, it was assumed that the Trichomatie placed through a gene duplication and subsequent mutation of the genes in the Old World monkeys.
However, for example in squirrel monkeys occurs the gene for the long-wavelength receptor in three alleles, the sensitivity of the human M and L cones is between the. Since the coding is also carried on the X chromosome, all males are dichromats. About two thirds of females are trichromats, because composed by her two X chromosomes, the possibility of having two different alleles. Since the pigments of the cones are almost identical for new and old world monkeys, is now believed that the mutation first occurred and split into at least two alleles, and after separation of the continents occurred, the gene duplication in the Old World monkeys.