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Human color vision is trichromatic and requires the normal function of three classes of cones responding to wavelengths of approximately 420nm (blue cones), 530 nm (green cones), and 560 nm (red cones). Dichromatic color vision discussed here is based on responses of red and green cones whose pigments are generated from contiguous gene regions on the X chromosome encoding OPN1MW (green pigment), and OPN1LW (red pigment).
The degree of color deficiency is variable and some males are so mildly affected that they are unaware of any defect until tested. The human eye is capable of seeing about a million colors which is made possible in part by the wide range of comparative signal outputs from the three classes of cones. In addition, the ratio of red and green cones varies among individuals and these factors collectively influence how each individual interprets the spectrum of wavelengths that enter the eye. The phenotype of red-green color blindness is highly variable.
Four subclasses of red-green color vision defects are recognized:
Protanopia – only blue and green cones are functional (1 percent of Caucasian males)
Deuteranopia – only blue and red cones are functional (1 percent of Caucasian males)
Protanomaly – blue and some green cones are normal plus some anomalous green-like cones (1 percent of Caucasian males)
Deuteranomaly – normal blue and some red cones are normal plus some anomalous red-like cones (5 percent of Caucasian males)
There are no systemic abnormalities.
Red-green color perception is based on gene products called opsins which, combined with their chromophores, respond to photons of specific wavelengths. The OPN1LW and OPN1MW genes reside in a cluster with a head-to-tail configuration on the X chromosome at Xq28. Red-green color vision defects are therefore inherited in an X-linked recessive pattern. There is a single gene for the red cone opsin but there are multiple ones for the green pigment. Only the red gene and the immediately adjacent green pigment gene are expressed. All are under the control of a master switch called the locus control region, LCR.
These DNA segments undergo relatively frequent unequal crossovers which can disrupt the color sensitivity of the gene products so that red-green colorblindness in some form is the most common type of anomalous color vision. It is found in approximately 8% of males and perhaps 0.5% of females.
No treatment is available for red-green color blindness although appropriately tinted lenses may enhance the perception of certain shades for specific tasks.