Dichromatic vision is a condition where color perception is derived from two distinct types of photoreceptor cells. In contrast, most humans have trichromatic vision, which uses three of these cell types. This difference means individuals with dichromacy see a limited range of colors, not a complete absence of color vision.
The Mechanics of Dichromatic Sight
Color perception begins in the retina, which contains photoreceptor cells: rods for detecting brightness and cones for color information in bright light. Each cone type contains a different light-sensitive pigment, making it sensitive to different wavelengths of light.
The S-cones are most responsive to short-wavelength light (blue), M-cones to medium-wavelength light (green), and L-cones to long-wavelength light (red). The brain compares signals from these three cone types to generate the full color spectrum. Dichromacy occurs when one of these cone types is missing or non-functional, limiting vision to the information from the remaining two.
Forms of Human Dichromacy
Protanopia results from the absence of L-cones, which are sensitive to red light. Individuals with this condition have difficulty distinguishing between red and green hues, and their perception of red is significantly dimmed. Certain shades of orange, yellow, and green may appear as a similar yellowish color. This sex-linked condition affects about 1% of males.
A different form of red-green color deficiency is deuteranopia, caused by the absence of M-cones responsible for detecting green light. Like protanopia, this condition leads to confusion between red and green shades. Deuteranopia is the most common form of dichromacy, and both it and protanopia are considered “red-green colorblindness.”
A much rarer form of dichromacy is tritanopia, characterized by the absence of S-cones. Individuals with tritanopia have difficulty distinguishing between blue and yellow and tend to see the world in shades of red, green, and gray. Unlike the other forms, the gene for the S-cone is on a different chromosome, so tritanopia is not sex-linked and occurs with very low frequency.
Dichromacy Across Species
While dichromacy is classified as a vision deficiency in humans, it is the standard for many other mammals. Animals such as dogs, cats, and bulls are natural dichromats. Their eyes contain two types of cone cells sensitive to blue and yellow-green wavelengths of light.
For these animals, the world is not seen in black and white but in a range of blues, yellows, and grays. They are unable to distinguish between reddish and greenish colors, similar to a human with deuteranopia. This vision is well-suited to their ecological niches, where detecting movement and contrast is more important than discerning a wide array of colors.