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Although the deep blue eyes of some people such as Elizabeth Taylor can appear purple or violet at certain times, "true" violet-colored eyes occur only due to albinism. [73] [74] [75] Eyes that appear red or violet under certain conditions due to albinism occur in less than 1 percent of the world's population. [70]
The four pigments in a bird's cone cells (in this example, estrildid finches) extend the range of color vision into the ultraviolet. [1]Tetrachromacy (from Greek tetra, meaning "four" and chroma, meaning "color") is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye.
The eyes of albino animals appear red because the colour of the red blood cells in the retina can be seen through the iris, which has no pigment to obscure this. Some albino animals may have pale-blue eyes due to other colour generating processes. Albino vertebrates exposed to intense light typically lose photoreceptors due to apoptosis. [28]
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene [5] and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rods.
Carotenoids are the most common group of pigments found in nature. [18] Over 600 different kinds of carotenoids are found in animals, plants, and microorganisms. Marine animals are incapable of making their own carotenoids and thus rely on plants for these pigments. Carotenoproteins are especially common among marine animals.
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Researchers studying the opsin genes responsible for color-vision pigments have long known that four photopigment opsins exist in birds, reptiles and teleost fish. [3] This indicates that the common ancestor of amphibians and amniotes (≈350 million years ago) had tetrachromatic vision — the ability to see four dimensions of color.