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Half of the Women See More Colors Than the Rest of the People Do
Tetrachromacy
By Stefan Anitei, Science Editor
June 26th, 2007, 18:16 GMT
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Normally, people have three types of cone cells for daylight, for detecting different colors. But some women can see extra colors as they have four types of cone cell receptors. They are called tetrachromats. Compared to them, we all are color blind.

The first tetrachromat woman was discovered by researchers at Cambridge University in 1993. This is perhaps the most remarkable human mutation ever detected. The fact that all tetrachromats are female intrigued scientists. Now two scientists, working separately, want to investigate systematically for tetrachromats to clarify more about their existence and how they detect colors.

All mammals descended from nocturnal tree dwellers, which were colorblind, but the line of primates had more advantages in developing color vision for finding fruit food. Human color vision is based on three forms of iodopsin (color pigments), each sensitive to a different light wavelength and is found in a different cone type. When a different cone type is stimulated, the brain reads it as a particular color.

The three iodopsins respond to red, green and blue; all the other colors are their combinations. Like all pigments, iodopsins are proteins encoded by DNA genes. The genes encoding the "red" and "green" iodopsins are located on the X sex chromosome, while the "blue" iodopsin is on a non-sexual chromosome.

That's why color-blindness mostly affects men: 8% of the Caucasian males; while under 0.5 % of Americana women present it. Women have X chromosomes: one from the mother and one the father, while men have just one X chromosome from the mother and an Y sex chromosome from the father (this one does not contain any iodopsin gene).

X chromosomes can be a "green" iodopsine or a slightly shifted "green" iodopsine, and a "red" iodopsine and a shifted "red" iodopsine. That's why a woman can carry 5 types of iodopsins: these four plus "blue", while a man just three (a green type, a red one plus blue).

A recent paper by Kimberly Jameson, Susan Highnote and Linda Wasserman of the University of California, San Diego, showed that up to 50 % of women carry 4 types of iodopsins and can employ their extra pigments in "contextually rich viewing circumstances".

For example, when looking at a rainbow, these females can segment it into about 10 different colors, while trichromat (with three iodopsins) people can see just seven: red, orange, yellow, green, blue, indigo and violet. For tetrachromat women, green was found to be assigned in emerald, jade, verdant, olive, lime, bottle and 34 other shades.

Still, the birds' abilities are even superior. Pigeons have five color receptors (and five types of cell receptors) and can process visual information up to 10 times faster than human beings. While we see a smooth TV image in real movement and color, they will see dull flickering lights.

Tetrachromats species are encountered among birds, insects, jumping spiders, reptiles, and amphibians, but no mammal is known to posses this. Some of them detect UV light.

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