Color vision deficiency (CVD), often incorrectly called color blindness, is a condition involving a reduced ability to distinguish between certain shades or colors, not a total absence of color perception. The public assumption that CVD means seeing the world strictly in black and white is a widespread myth. Most individuals with this condition retain a significant capacity to see a spectrum of color, though their experience of hue is distinctly different from the average person. The science of color perception and its deficiencies reveals a complex system of photoreceptors in the eye that can be partially or fully impaired.
The Reality of Color Vision Deficiency
The vast majority of people with a color vision deficiency do not see a monochrome world of only black, white, and gray. Instead, they experience a narrowing of the color spectrum that makes distinguishing between specific hues challenging. The severity of the deficiency can range from mild to significant, affecting the perception of color contrast and saturation. This common form of CVD is known as anomalous trichromacy, indicating that the individual possesses all three types of cone cells but one type functions abnormally. While a person with typical vision can see an estimated one million distinct shades, those with anomalous trichromacy may perceive a drastically reduced number of hues.
Understanding Red-Green Color Deficiencies
The most prevalent types of color vision deficiency are those affecting the red and green color spectrum, accounting for roughly 99% of all cases of congenital CVD. These deficiencies are directly related to the light-sensitive pigments, or opsins, in the long-wavelength (red) and medium-wavelength (green) cone cells of the retina.
Deuteranomaly
The most common specific form is Deuteranomaly, where the medium-wavelength (green) cones are the ones with an abnormal sensitivity. Individuals with Deuteranomaly perceive greens as shifted toward red, making the distinction between red, green, brown, and orange hues particularly difficult. This condition is the mildest and most frequent form of CVD, affecting approximately six percent of men worldwide.
Protanomaly
Protanomaly is the second main type of red-green deficiency, characterized by a reduced sensitivity in the long-wavelength (red) cones. For those with Protanomaly, red, orange, and yellow appear shifted toward green and are also perceived as less bright than they are to a person with typical vision. This reduction in perceived brightness for red light is a significant difference between Protanomaly and Deuteranomaly.
Dichromacy
The more severe forms of red-green CVD are protanopia and deuteranopia, known collectively as dichromacy, where one type of cone cell is completely non-functional. People with dichromacy see the world in only two primary color dimensions, often perceiving a world dominated by blues and yellows. Even in these cases, the vision is not black and white, but rather a reduced color palette.
When People Truly See in Shades of Gray
The idea that color deficient people see only in black and white is based on a rare condition called Achromatopsia, or monochromacy, which is the only type that accurately matches the common misconception. Achromatopsia is a genetic disorder where the cone cells, the photoreceptors responsible for color vision, are either completely non-functional or entirely absent. This condition is very rare, affecting an estimated one in 30,000 people globally.
Individuals with complete achromatopsia rely entirely on their rod cells, which are the photoreceptors used for low-light vision and are only sensitive to light and dark, resulting in a true perception of the world in shades of gray. This complete lack of color perception is often accompanied by other visual impairments, including extreme light sensitivity, called photophobia, and significantly reduced sharpness of vision.
Tritanomaly
Another rare form of CVD is Tritanomaly, a blue-yellow deficiency that involves the short-wavelength (blue) cone cells. This condition makes it difficult to distinguish between blue and green, as well as between yellow and red. Tritanomaly is much less common than the red-green types and is often acquired later in life, though it can also be inherited.
The Genetics Behind Color Vision Differences
The most common forms of color vision deficiency are inherited and occur due to differences in the genes that produce the light-sensitive opsin proteins in the cone cells. The genes for the red and green opsins are located on the X chromosome. Because red-green CVD is inherited in an X-linked recessive pattern, males, who have only one X chromosome, express the trait. Females have two X chromosomes, and the presence of one functional gene is usually enough to mask the effect of the affected gene. This genetic mechanism explains why red-green color deficiency affects approximately 8% of men, compared to less than one percent of women.
The S-cone opsin gene, responsible for blue light sensitivity, is located on chromosome 7. This is why blue-yellow deficiencies are inherited through a different, typically autosomal dominant, pattern. The three types of cone cells—short (blue), medium (green), and long (red) wavelength—normally work together to allow the perception of a full range of colors.