Color vision deficiency (CVD), often called “color blindness,” impacts the ability to distinguish between certain shades or colors. This difference in perception stems from issues with the light-sensitive cells in the eye’s retina. While CVD is overwhelmingly more common in men, it is a misconception that women are immune to the condition. The underlying science explains why the deficiency affects men so disproportionately and the specific, rare circumstances under which women are affected.
Understanding Color Vision Deficiency
The ability to see color relies on specialized cells within the retina called cones, which contain light-sensitive pigments. Humans typically have three types of cones, each tuned to absorb light at different wavelengths corresponding roughly to red, green, and blue. Color vision deficiency occurs when one or more of these cone types are either absent or function incorrectly, leading to a decreased ability to perceive color variations.
The vast majority of inherited cases involve the red-green spectrum, including deuteranomaly (faulty green cones) and protanomaly (faulty red cones). Rarer forms include blue-yellow deficiency (tritanomaly) and the extremely rare monochromacy, or total color blindness, where a person sees only shades of gray. Diagnostic tools like the Ishihara test, which uses plates of colored dots to hide numbers or patterns, are commonly used to identify these deficiencies.
The Genetic Explanation for Rarity in Women
The reason CVD is so uncommon in women compared to men lies in the pattern of inheritance, which is primarily X-linked recessive. The genes responsible for producing the red and green light-sensitive pigments (OPN1LW and OPN1MW) 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 she inherits one faulty X chromosome and one healthy one, the healthy gene usually compensates. This makes her a “carrier” who does not experience the deficiency but can still pass the gene to her children.
A male, however, has no second X chromosome to provide a healthy backup copy of the gene. If the single X chromosome he inherits carries the defective color vision gene, he will be affected by the condition. This difference in chromosome pairs is the fundamental reason for the significant disparity in prevalence between the sexes.
How Women Become Affected Individuals
Women can be affected by inherited color vision deficiency, but the necessary genetic scenario is highly improbable due to the protective mechanism of having two X chromosomes. For a woman to inherit the common X-linked red-green CVD, she must receive a defective X chromosome from both her mother and her father. This requires her father to be affected by CVD and her mother to be at least a carrier of the gene.
The rarity of this dual inheritance is reflected in the statistics: approximately 1 in 12 men are affected, while only about 1 in 200 women have the condition in populations of Northern European ancestry. The probability is different for blue-yellow CVD, which is caused by a gene on a non-sex chromosome, meaning it affects men and women equally, though it is much rarer overall.
In addition to inherited forms, women can acquire color vision deficiency later in life due to non-genetic factors. Damage to the retina or the optic nerve from disease or injury can impair color perception. Conditions such as diabetes, glaucoma, multiple sclerosis, and Alzheimer’s disease have been shown to cause acquired CVD.
Certain medications, including those used to treat heart conditions, high blood pressure, or infections, may also have side effects that affect color vision. Acquired CVD differs from the inherited form because it may affect only one eye or worsen over time, which is not the case with the stable, lifelong inherited deficiency.