Color blindness 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 (in females, a functional gene on only one of the two X chromosomes is sufficient to yield the needed 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.
The English chemist John Dalton published the first scientific paper on this subject in 1798, “Extraordinary facts relating to the vision of colours”, 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 more narrowly for deuteranopia alone.
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. And 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.
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 0.5 percent of women experience the common form of red-green color blindness. 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 often called S cones, M cones, and L cones, for short, medium, and long wavelength; but they are also often referred to as blue cones, green cones, and red cones, respectively.
Color blindness can be inherited. It is most commonly inherited from mutations on the X chromosome but the mapping of the human genome has shown there are many causative mutations—mutations capable of causing color blindness originate from at least 19 different chromosomes and 56 different genes (as shown online at the Online Mendelian Inheritance in Man (OMIM) database at Johns Hopkins University). Two of the most common inherited forms of color blindness are protanopia, and deuteranopia. One of the common color vision defects is the red-green deficiency which is present in about 8 percent of males and 0.5 percent of females of Northern European ancestry.
Some of the inherited diseases known to cause color blindness are:
Inherited color blindness can be congenital (from birth), or it can commence in childhood or adulthood. Depending on the mutation, it can be stationary, that is, remain the same throughout a person’s lifetime, or progressive. As progressive phenotypes involve deterioration of the retina and other parts of the eye, certain forms of color blindness can progress to legal blindness, i.e., an acuity of 6/60 or worse, and often leave a person with complete blindness.
Color blindness always pertains to the cone photoreceptors in retinas, as the cones are capable of detecting the color frequencies of light.
Other causes of color blindness include brain or retinal damage caused by shaken baby syndrome, accidents and other trauma which produce swelling of the brain in the occipital lobe, and damage to the retina caused by exposure to ultraviolet light (10–300 nm). Damage often presents itself later on in life.
Color blindness may also present itself in the spectrum of degenerative diseases of the eye, such as age-related macular degeneration, and as part of the retinal damage caused bydiabetes. Another factor that may affect color blindness includes a deficiency in Vitamin A.
Achromatopsia is strictly defined as the inability to see color. Although the term may refer to acquired disorders such as cerebral achromatopsia also known as color agnosia, it typically refers to congenital color vision disorders.
Protanopia, deuteranopia, protanomaly, and deuteranomaly are widely common inherited color blindness that affects a substantial portion of the human population, in which those affected have difficulty with discriminating red and green hues due to the absence of the red or green retinal photoreceptors. It is sex-linked: genetic red–green color blindness affects males much more often than females, because the genes for the red and green color receptors are located on the X chromosome, of which males have only one and females have two. Females (46, XX) are red–green color blind only if both their X chromosomes are defective with a similar deficiency, whereas males (46, XY) are color blind if their single X chromosome is defective.
Those with tritanopia and tritanomaly have difficulty discriminating between bluish and greenish hues, as well as yellowish and reddish hues.
Color blindness involving the inactivation of the short-wavelength sensitive cone system is called tritanopia or, loosely, blue–yellow color blindness. The tritanopes neutral point occurs near a yellowish 570 nm; green is perceived at shorter wavelengths and red at longer wavelengths. Mutation of the short-wavelength sensitive cones is called tritanomaly.
The Ishihara color test, which consists of a series of pictures of colored spots, is the test most often used to diagnose red–green color deficiencies. A figure is embedded in the picture as a number of spots in a slightly different color, and can be seen with normal color vision, but not with a particular color defect. The full set of tests has a variety of figure/background color combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described above, is also used in diagnosing anomalous trichromacy.