Dichromacy

Dichromacy is a type of color vision deficiency where a person has only two types of functional color receptor cones in the retina, rather than the normal three. This results in an inability to distinguish between certain color pairs. There are three main types: Protanopia, Deuteranopia, and Tritanopia.

Protanopia is one of the most common types of dichromacy. Patients do not have red cone cells in the retina. They are unable to perceive red and have significant difficulty distinguishing greens. Singly, blue and yellow are easily spotted, but combined red-green shades appear gray. Affects approximately 1% of males.

Deuteranopia is the most common form of dichromacy — affecting about 1% of males. The green cone is absent. People with deuteranopia have difficulty distinguishing between red and green shades. Unlike protanopia, the perceived brightness of colors is relatively normal. The world looks mostly in yellows, blues, and beiges.

Tritanopia is a very rare type of dichromacy where the blue cone is missing. People with tritanopia confuse blue with green, and yellow with violet or red. This type is different from protan and deutan deficiencies in that it is not sex-linked — it affects males and females equally. Tritanopia affects less than 0.01% of people.