Color vision deficiency, commonly known as color blindness, is a condition where a person perceives colors differently than someone with typical color vision. This is usually not a form of true blindness, but rather an inability to distinguish between certain shades or hues, most often involving red and green. Color perception begins in the retina, where specialized cone photoreceptor cells convert light into signals sent to the brain. Color deficiency results from a defect in these cone cells, which are responsible for detecting color under normal light conditions. There are typically three types of cones, sensitive to short, medium, or long wavelengths of light, corresponding roughly to blue, green, and red.
Prevalence in Women and Sex Differences
The most common forms of inherited color vision deficiency affect women at a significantly lower rate than men. For populations of Northern European ancestry, red-green color deficiency affects approximately 1 in 12 males (about 8%). In contrast, the condition is rare in women, occurring in about 1 in 200, or just 0.5% of the female population. The statistical reality that color deficiency is nearly 16 times more common in men establishes the sex-linked pattern of inheritance.
The Mechanism of X-Linked Inheritance
The vast difference in prevalence is explained by the X-linked recessive inheritance pattern of the genes responsible for red-green color vision. The instructions for creating the light-sensitive pigments in the red and green cone cells are located on the X chromosome. Females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). Because the trait is recessive, a female must inherit a defective gene on both X chromosomes to express the condition.
If a female inherits one defective gene, the functional gene on the healthy chromosome is typically sufficient to ensure normal color vision, making her a carrier. A male is affected if his single X chromosome carries the defective gene, as he lacks a second X chromosome to provide a healthy copy. For a female to be color deficient, her father must be color deficient and her mother must be at least a carrier. The genes involved (OPN1LW and OPN1MW), which encode the red and green cone pigments, are highly similar and tightly linked on the X chromosome.
Specific Types of Inherited Color Vision Deficiency
Inherited color vision deficiencies are categorized by the specific cone cell type affected, leading to two main groups: red-green and blue-yellow defects. Red-green defects are divided into protan and deutan types, affecting the long-wavelength (red) and medium-wavelength (green) cones, respectively. When one cone type is absent altogether, the condition is called dichromacy, such as protanopia (missing red cones) or deuteranopia (missing green cones).
A less severe form, anomalous trichromacy, occurs when all three cone types are present, but one has an abnormal light-sensing pigment, causing a shift in color perception. Deuteranomaly, a partial deficiency of the green cone pigment, is the most common type of color vision deficiency overall.
Blue-yellow defects, known as tritan defects, are much rarer and involve issues with the short-wavelength (blue) cones. Unlike red-green defects, tritanopia and tritanomaly are inherited as an autosomal trait, meaning the responsible gene (OPN1SW) is located on a non-sex chromosome. This results in the prevalence of tritan defects being roughly equal between males and females. The most severe form, achromatopsia or monochromacy, involves a complete absence of color perception, leaving vision only in shades of gray.
Acquired Color Vision Deficiencies
While most color vision issues are inherited, the condition can also be acquired later in life due to various external factors and diseases. Unlike genetic forms, which affect both eyes equally and remain stable, acquired deficiencies can vary over time and may affect only one eye. These non-genetic causes often involve damage to the retina, optic nerve, or parts of the brain responsible for processing visual information.
A wide range of eye diseases, including glaucoma, cataracts, age-related macular degeneration, and diabetic retinopathy, can cause acquired color deficiency. Systemic illnesses such as multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease, or exposure to certain industrial chemicals and medications, are also linked to changes in color vision. Acquired deficiencies tend to affect men and women with similar frequency and often present as blue-yellow defects, contrasting with the predominantly red-green nature of inherited conditions.