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Color blindness, or color vision deficiency, is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. Color blindness affects a significant percentage of the population. There is no actual blindness but there is a deficiency of color vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. This type of color blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome, whereas females have two and a functional gene on only one of the X chromosomes is sufficient to yield the necessary photopigments. Color blindness can also be produced by physical or chemical damage to the eye, the optic nerve, or parts of the brain. For example, people with achromatopsia suffer from a completely different disorder, but are nevertheless unable to see colors. Color blindness is usually classified as a mild disability, however there are occasional circumstances where it can give an advantage. Some studies conclude that color blind people are better at penetrating certain color camouflages. Such findings may give an evolutionary reason for the high prevalence of red–green color blindness. There is also a study suggesting that people with some types of color blindness can distinguish colors that people with normal color vision are not able to distinguish. ==Background== (詳細はJohn Dalton in 1798 after the realization of his own color blindness. Because of Dalton's work, the general condition has been called ''daltonism'', although in English this term is now used only for deuteranopia. Color blindness affects a large number of individuals, with protanopia and deuteranopia being the most common types.〔 In individuals with Northern European ancestry, as many as 8 percent of men and 0.4 percent of women experience congenital color deficiency. The typical human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cone cells, each containing a different pigment, which are activated when the pigments absorb light. The spectral sensitivities of the cones differ; one is most sensitive to short wavelengths, one to medium wavelengths, and the third to medium-to-long wavelengths within the visible spectrum, with their peak sensitivities in the blue, green, and yellow-green regions of the spectrum, respectively. The absorption spectra of the three systems overlap, and combine to cover the visible spectrum. These receptors are known as short (S), medium (M),and long (L) wavelength cones, but are also often referred to as blue, green, and red cones, although this terminology is inaccurate. The receptors are each responsive to a wide range of wavelengths. For example, the long wavelength, "red", receptor has its peak sensitivity in the yellow-green, some way from the red end (longest wavelength) of the visible spectrum. The sensitivity of normal color vision actually depends on the overlap between the absorption ranges of the three systems: different colors are recognized when the different types of cone are stimulated to different degrees. Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing the wavelength causes the other two cone systems to be increasingly stimulated, causing a gradual change in hue. Many of the genes involved in color vision are on the X chromosome, making color blindness much more common in males than in females because males only have one X chromosome, while females have two. Because this is an X-linked trait, an estimated 2–3% of women have a 4th color cone and can be considered tetrachromats, although it is not clear that this provides an advantage in color discrimination. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Color blindness」の詳細全文を読む スポンサード リンク
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