Color Blindness Knowledge Base
Color blindness background
Color blindness occurs when a single type of color-receptor cone is missing from the retina,
due to a genetic defect, rendering the person unable to distinguish some colors from others.
There are three main types of color blindness-green, red and rarely, blue.
We also have black and white receptors. They are more sensitive than the color receptors, that is why we have poor color perception in the dark.
The normal 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 cones, each containing a different pigment. The cones are activated when the pigments absorb light. The absorption spectra of the cones differ; one is maximally sensitive to short wavelengths, one to medium wavelengths, and the third to long wavelengths (their peak sensitivities are in the blue, yellowish-green, and yellow regions of the spectrum, respectively). The absorption spectra of all three systems cover much of the visible spectrum, so it is not entirely accurate to refer to them as "blue", "green" and "red" receptors, especially because the "red" receptor actually has its peak sensitivity in the yellow.
The sensitivity of normal color vision actually depends on the overlap between the absorption spectra of the three systems: different colors are recognized when the different types of cone are stimulated to different extents. Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing 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 (male XY; female XX), making color blindness more common in males than in females. Most color perception defects are for red or green or both. About 10% of males have a color perception defect, but this is rare in females. If you're colour blind you're born with it and it will stay the same forever. But no matter how old you get, you cannot become colour blind, you have to be born with it.
Causes of color blindness
Color blindness can be inherited genetically. Some people believe, incorrectly, that it is only ever 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 many different genes (as shown online at the Online Mendelian Inheritance in Man (OMIM) database at Johns Hopkins University). Cone dystrophy, Cone-rod dystrophy, Achromatopsia (aka Rod Monochromatism, aka Stationery Cone Dystrophy, aka Cone Dysfunction Syndrome), Blue cone monochromatism, Retinitis pigmentosa (initially affects rods but can later progress to cones and therefore color blindness), Diabetes, Age-Related Macular degeneration, Retinoblastoma, Leber's congenital amaurosis - These are some of the inherited diseases known to cause color blindness.
Those with protanopia, deuteranopia, protanomaly, and deuteranomaly have difficulty with discriminating red and green hues. Genetic red-green color blindness affects men much more often than women, because the genes for the red and green color receptors are located on the X chromosome, of which men have only one and women have two. Such a trait is called sex-linked. 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.
The gene for red-green color blindness is transmitted from a color blind male to all his daughters who are heterozygote carriers and are usually unaffected. In turn, a carrier woman has a fifty percent chance of passing on a mutated X chromosome region to each of her male offspring. The sons of an affected male will not inherit the trait from him, since they receive his Y chromosome and not his (defective) X chromosome. Should an affected male have children with a carrier or colorblind woman, their daughters may be colorblind by inheriting an affected X chromosome from each parent.
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 our retina as the cones are capable of detecting the color frequencies of light we perceive.
About 5–8 percent of males, but less than 1 percent of females, are color blind in some way or another, whether it be one color, a color combination, or another mutation. The reason males are at a greater risk of inheriting an X linked mutation is because males only have one X chromosome (XY, with the Y chromosome being significantly shorter than the X chromosome), and females have two (XX); if the women inherit a normal X chromosome in addition to the one which carries the mutation, they will not display the mutation, while men have no 'spare' normal chromosome to override the chromosome which carries the mutation. If 5% of variants of a given gene are defective, the probability of a single copy being defective is 5%, but the probability that two copies are both defective is 0.05 ? 0.05 = 0.0025, or just 0.25%.
Shaken Baby Syndrome (this can cause retina and brain damage and therefore cause color blindness), accidents and other trauma (swelling of the brain in the occipital lobe), and UV damage to the retina (from not wearing appropriate protection). Most UV damage is caused during childhood and this form of retinal degeneration is the leading cause of blindness in the world. Damage often presents itself later on in life.
Types of color blindness
There are many types of color blindness. The most common are red-green hereditary (genetic) photoreceptor disorders, but it is also possible to acquire color blindness through damage to the retina, optic nerve, or higher brain areas. Higher brain areas implicated in color processing include the parvocellular pathway of the lateral geniculate nucleus of the thalamus, and visual area V4 of the visual cortex. Acquired color blindness is generally unlike the more typical genetic disorders. For example, it is possible to acquire color blindness only in a portion of the visual field but maintain normal color vision elsewhere. Some forms of acquired color blindness are reversible. Transient color blindness also occurs (very rarely) in the aura of some migraine sufferers.
The different kinds of inherited color blindness result from partial or complete loss of function of one or more of the different cone systems. When one cone system is compromised, dichromacy results. The most frequent forms of human color blindness result from problems with either the middle or long wavelength sensitive cone systems, and involve difficulties in discriminating reds, yellows, and greens from one another. They are collectively referred to as "red-green color blindness", though the term is an over-simplification and is somewhat misleading. Other forms of color blindness are much more rare. They include problems in discriminating blues from yellows, and the rarest forms of all, complete color blindness or monochromacy, where one cannot distinguish any color from grey, as in a black-and-white movie or photograph.
Clinical forms of color blindness
Based on clinical appearance, color blindness may be described as total or partial. Total color blindness is much less common than partial color blindness. There are two major types of color blindness: those who have difficulty distinguishing between red and green, and those who have difficulty distinguishing between blue and yellow.
(a) Total color blindness
Treatment for color deficiencies
There is generally no treatment to cure color deficiencies. However, certain types of tinted filters and contact lenses may help an individual to distinguish different colors better. Optometrists can supply a singular red-tint contact lens to wear in the dominant eye. This may enable the wearer to pass some color blindness tests for certain occupations, but the wearer is not better suited. The effect of wearing such a device is akin to wearing red/blue 3D glasses and can take some time getting used to as certain wavelengths can "jump" out and be overly represented. Additionally, computer software has been developed to assist those with visual color difficulties.