Colour blindness

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Most Homo sapiens have three varieties of colour-sensitive cells - cones - in the retina (some humans and other species of animals have none, two, three or four). These have arisen by reduplication of the gene for the protein which holds a stressed Vitamin A residue ready to be altered by a photon and its subsequent mutation leading to different peak sensitivity wavelengths for each protein. Each cone cell turns on one of these genes, randomly, and thus populations of cones with different sensitivities form in the retina. The commonest deficits in man are X-linked.

If one of those genes is an allele for an ineffective version of the protein then each cone expressing that gene will fail to work, and poor discrimination of light of the wavelength they would otherwise be tuned to will result. The commonest defects are such that good design practice would tend to use red and blue with shading tested against gray scale, rather than red and green and colours representing intermediate values having all the same gray scale values.

The degree of sensitivity is variable and the maximum sensitivity is only required for certain critical tasks - recognising faint lights at sea, wiring complex coloured looms into electronic equipment and the like. The type of colour blindness can be tested with Ishihara plates but a Farnsworth Lantern test is required for some purposes. People who fail the Ishihara plates may well pass the lantern test. There are other more refined tests such as the HRR test and the Farnsworth-Munsell 100 hue test[1].

In man the abnormalities may have occupational implications, such as for commercial airline pilots and graphic designers. This includes for doctors[2] and some medical schools internationally screen for it. It is also possible to acquire colour blindness as a consequence of some genetic forms of blindness and conditions such as macular degeneration or exposure to chemicals or medicines such as hydroxychloroquine. Use of say ethambutol would be expected to cause over 10% to have a colour perceptual problem[3]. Indeed a significant proportion of colour blindness may be acquired due to the high prevalence of causative disease in the elderly. Perception of colour also depends upon CNS processing of the information and it is now known that 'normal' humans can perceive the colours of an object as quite different depending upon how their brains process cues as to illumination.

Epidemiology

Incidence is difficult to ascertain because of the higher frequency of the condition in those of European ancestry, and sub-populations world wide with founder effects. About a quarter of anomalous trichromats will be unaware of their condition until formally tested.

Genetic abnormalities in colour perception
Type Males Females Features
Dichromacy 2.4% 0.03%
Protanopia 1.3% 0.02% red deficient: L cone absent
Deuteranopia 1.2% 0.01% green deficient: M cone absent
Tritanopia Incidence 1:13,000, blue deficient: S cone absent
Anomalous trichromacy 6.0% 0.37%
Protanomaly 1,0%[4] 0.02% X-linked, red deficient: L photopigment defect
Deuteranomaly 5.0%[4] 0.35% X-linked, green deficient: M photopigment defect
Tritanomaly Incidence 0.0001%, blue deficient: S photopigment defect
Tetrachromacy Very rare Perception of wider gambit of colours than normal
Monochromacy
Rod monochromacy 1 in 30,000 with poor visual acuity, marked photophobia, congenital nystagmus, and complete colorblindness
Blue-cone monochromacy 1 in 100,000, At birth reduced visual acuity, pendular nystagmus, and photophobia
Green-cone monochromacy (GCM, M-cone monochromacy) 1 in 1,000,000
Red-cone monochromacy ((RCM, L-cone monochromacy) 1 in 1,000,000

References