Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cells in the eye. Organisms with tetrachromacy are called tetrachromats.
In tetrachromatic organisms, the sensory color space is four-dimensional, meaning that to match the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four primary colors.
Tetrachromacy is demonstrated among several species of birds, fish, amphibians, reptiles and insects. It was also the normal condition of most mammals in the past; a genetic change made the majority of species of this class eventually lose two of their four cones.
The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells which are lower intensity light receptors) with different absorption spectra. This means the animal may see wavelengths beyond those of a typical human being's eyesight, and may be able to distinguish colors that to a human appear to be identical. Species with tetrachromatic color vision have a small physiological advantage over rival species.
Some species of birds such as the Zebra Finch and the Columbidae use the ultraviolet wavelength 300–400 nm specific to tetrachromatic color vision as a tool during mate selection and foraging. When selecting for mates, ultraviolet plumage and skin coloration show a high level of selection. A typical bird eye will respond to wavelengths from about 300 to 700 nm. In terms of frequency, this corresponds to a band in the vicinity of 430–1000 THz.
Bird’s eyes are tetrachromats, but their retina cone cells are much more complex than those of humans:
Birds have many more cones than humans and thus can see colors better than humans. Birds' photopigments are sensitive to four or five peak wavelengths, and thus birds are much more sensitive to colors than humans are.
Foraging insects can see wavelengths that flowers reflect (ranging from 300 nm to 700 nm). Pollination being a mutualistic relationship, foraging insects and plants have coevolved, both increasing wavelength range: in perception (pollinators), in reflection and variation (flower colors). Directional selection has led plants to display increasingly diverse amounts of color variations extending into the ultraviolet color scale, thus attracting higher levels of pollinators. Some pollinators may use tetrachromatic color vision to increase and maintain a higher foraging success rate over their trichromatic competitors.
Humans, apes and Old World monkeys normally have three types of cone cells and are therefore trichromats (animals with three types of cones). However, at low light intensities the rod cells may contribute to color vision, giving a small region of tetrachromacy in the color space; human rod cells' sensitivity is greatest at a blueish-green wavelength.
In humans, two cone cell pigment genes are on the sex X chromosome, the classical type 2 opsin genes OPN1MW and OPN1MW2. It has been suggested that as women have two different X chromosomes in their cells, some of them could be carrying some variant cone cell pigments, thereby possibly being born as full tetrachromats and having four simultaneously functioning kinds of cone cells, each type with a specific pattern of responsiveness to different wavelengths of light in the range of the visible spectrum. One study suggested that 2–3% of the world's women might have the type of fourth cone whose sensitivity peak is between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. Another study suggests that as many as 50% of women and 8% of men may have four photopigments and corresponding increased chromatic discrimination compared to trichromats. In June 2012, after 20 years of study of women with four types of cones (non-functional tetrachromats), neuroscientist Dr. Gabriele Jordan identified a woman (subject cDa29) who could detect a greater variety of colors than trichromats could, corresponding with a functional tetrachromat (or true tetrachromat). 
Variation in cone pigment genes is widespread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red/green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female new-world monkeys trichromatic vision.
In humans, preliminary visual processing occurs in the neurons of the retina. It is not known how these nerves would respond to a new color channel, that is, whether they could handle it separately or just combine it in with an existing channel. Visual information leaves the eye by way of the optic nerve; it is not known whether the optic nerve has the spare capacity to handle a new color channel. A variety of final image processing takes place in the brain; it is not known how the various areas of the brain would respond if presented with a new color channel.
Mice, which normally have only two cone pigments, can be engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination, arguing against some of these obstacles; however, the original publication's claims about plasticity in the optic nerve have also been disputed.
Humans cannot see ultraviolet light directly because the lens of the eye blocks most light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. The photoreceptor cells of the retina are sensitive to near ultraviolet light and people lacking a lens (a condition known as aphakia) see near ultraviolet light (down to 300 nm) as whitish blue, or for some wavelengths, whitish violet, probably because all three types of cones are roughly equally sensitive to ultraviolet light, but blue cones a bit more.
Tetrachromacy may also enhance vision in dim lighting.