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)
Blue color blindness (tritanopia; 190900) is the result of mutations in the OPN1SW gene on chromosome 7. ERG flicker responses can be used to define the type and nature of the cone defects.