dichromacy

Dichromacy is a form of color vision deficiency in which the retina has only two functioning cone photopigments, rather than three. As a result, individuals with dichromacy perceive a reduced range of colors and have altered color discrimination compared with typical trichromatic vision.

There are three principal forms: protanopia, deuteranopia, and tritanopia. Protanopia is the absence of the long-wavelength (L) cones, deuteranopia is the absence of the medium-wavelength (M) cones, and tritanopia is the absence of the short-wavelength (S) cones. In protanopia and deuteranopia, red-green discrimination is impaired; in tritanopia, blue-yellow discrimination is affected.

Genetically, protanopia and deuteranopia are usually inherited as X-linked conditions, so they occur more commonly in men. Tritanopia is much rarer and inherited in an autosomal recessive pattern. Prevalence estimates vary, but dichromacy in its two common forms affects a small portion of the population; many affected individuals are unaware of the condition unless tested.

Diagnosis is typically made with standardized color vision tests, such as the Ishihara plates or more detailed assessments. The condition does not constitute complete blindness, but it can complicate tasks that rely on color cues, such as interpreting color-coded signals, choosing clothing, or color-matching for certain jobs.

There is no cure for dichromacy. Some individuals benefit from color-correcting lenses or glasses that enhance contrast for certain tasks, though these devices do not restore normal color perception. Adaptation and the use of alternative cues are common strategies.