Color vision deficiency (CVD), commonly known as color blindness, impairs the ability to distinguish between certain colors or shades. Eye color and color perception are governed by distinct biological processes and genetic locations. The two traits are not related in a meaningful way, and eye color does not influence the likelihood of color blindness.
How Eye Color is Determined
Eye color is a polygenic trait, influenced by the interaction of multiple genes. The color spectrum, from light blue to dark brown, is determined by the amount and distribution of melanin within the iris. Brown eyes have high concentrations of melanin, while blue and green eyes have less.
Lighter eye colors, such as blue, are not caused by blue pigment. They result from a structural color effect where low melanin content in the iris stroma causes light to scatter. The two most influential genes governing this pigmentation are OCA2 and HERC2, located on chromosome 15. Variations in HERC2 regulate the expression of OCA2, directly affecting melanin production and resulting in a lighter or darker eye shade.
The Biological Basis of Color Blindness
Color blindness (CVD) arises from issues with the photoreceptor cells in the retina called cone cells. A person with typical color vision, a trichromat, possesses three types of cones, each sensitive to different wavelengths of light: short (blue), medium (green), and long (red). Inherited color blindness occurs when one or more of these cone types are defective or absent, diminishing the ability to perceive color.
The most common forms are red-green color blindness, affecting up to 8% of males and 0.5% of females. The genes responsible for the L-cone and M-cone pigments are located on the X chromosome. Because males possess only one X chromosome, they are more susceptible to inheriting these X-linked recessive conditions. A much rarer form, blue-yellow color blindness (tritanopia), is caused by a mutation in the OPN1SW gene on chromosome 7.
The Lack of Correlation Between Pigment and Perception
The physical processes determining eye color and color perception operate independently. Eye color involves the production of melanin pigment in the iris, while color blindness involves the function of light-sensing cone cells in the retina. The genes controlling iris pigmentation (OCA2 and HERC2 on chromosome 15) are separate from the genes controlling cone cell function (primarily on the X chromosome and chromosome 7).
These genetic locations are physically separate, meaning the inheritance of one trait does not affect the inheritance of the other. The eye color mechanism is structural, involving light scattering and pigment concentration. Conversely, the color vision mechanism is functional, involving photopigment proteins converting light into neural signals. No biological link exists between eye shade and the likelihood of inherited color blindness.