Why does the iris in your eye have color?
The iris is that thin circular colored structure of the eye responsible for controlling the diameter and size of the pupil. The iris manages the amount of light entering the eye and reaching the retina. Muscles attached to the iris expand and contract the opening or aperture at the center of the iris, known as the pupil. The larger the pupil, the more light can enter the eye and reach the retina. It is an automatic process.
Eye color is determined by as many as six genes and is set by the amount, density, and type of pigment in the iris. For example, a green/blue eye color is located on chromosome 19. A brown eye and brown/blue eye color gene is located on chromosome 15. There are three pigment colors that determine the outward appearance of eye color: brown, yellow, and blue. Eye color is an inherited trait.
The most common eye color is brown, the least common is green. Babies of European ancestry are born with blue eyes. Melanin, the brown pigment that colors skin, hair, and eyes, has been not been developed yet.
The amount and type of melanin protein is encoded in the genes. If the iris gets a lot of melanin it will appear brown. Less melanin produces green, gray, or light brown. Very little melanin means blue eyes.
Changes in eye color do have medical implications. People with lighter iris color have a higher degree of age-related macular degeneration. Color change, especially yellowing of the whites of the eye, can indicate jaundice, cirrhosis, hepatitis, or malaria.
Scanning the retina has been an identification technique for several decades. Every person has a unique pattern of blood vessels in the back of the eye. An infrared beam scans the retina and the resulting pattern is compared to the persons’ scan already in the computer. Iris scanning is relatively new and is unimpeded by glasses or contacts.
There are a number of questionnaires and surveys that purport to link eye color to personality, intelligence, athletic ability, social skills, academic achievement, or compatibility. Like those astrology columns in the newspaper, it is all nonsense.