The human eye displays a spectrum of colors, from deep browns to vibrant greens and blues. This diversity sparks curiosity about whether eye color can naturally change, particularly from brown to green. Understanding the biological mechanisms that establish eye color offers insight into the stability of this unique human characteristic.
How Eye Color Is Determined
Eye color is primarily determined by the amount and distribution of a pigment called melanin within the iris, the colored part of the eye surrounding the pupil. Specialized cells called melanocytes produce melanin. The iris contains two main types of melanin: eumelanin, which provides a dark brown to black hue, and pheomelanin, which contributes to amber, yellow, or reddish-yellow tones.
The concentration of these melanin types dictates the visible eye color. Brown eyes, the most common eye color, result from a high concentration of eumelanin, which absorbs most light entering the eye. In contrast, blue eyes have very low levels of melanin in the iris, causing light to scatter, making them appear blue. Green eyes arise from a moderate amount of melanin, often a combination of low eumelanin and some pheomelanin, coupled with light scattering.
The genetic blueprint for eye color is complex, involving multiple genes rather than a single gene. While earlier models suggested a simple dominant-recessive inheritance, current understanding recognizes that at least eight to sixteen genes contribute to the final shade. Key genes like OCA2 and HERC2, located on chromosome 15, play roles in regulating melanin production and distribution, influencing the spectrum from brown to blue and green.
The Immutability of Adult Eye Color
For most adults, the actual color of the iris remains stable and does not naturally change from brown to green. Once the iris fully develops, by early childhood, the melanin content within its structure becomes fixed. This stability means that a true, permanent shift in iris pigmentation in adulthood does not occur.
While the eye’s intrinsic color does not change, individuals may perceive variations in their eye color due to temporary factors. Changes in lighting can alter how light interacts with the iris, making the color appear different. Pupil dilation, where the pupil expands or contracts to regulate light entry, can also influence perceived eye color by exposing more or less of the iris’s pigmented areas. The color of clothing or makeup can also create an optical illusion, making the eyes appear a slightly different hue, especially for lighter eye colors.
Circumstances That Influence Eye Appearance
Although a natural brown-to-green eye color change is not typical for adults, several circumstances can influence eye appearance or, in rare cases, cause color alteration. A common change occurs during infancy. Many babies are born with lighter eyes, often blue or gray, because their melanocytes have not yet produced enough melanin. As infants are exposed to light and their melanocytes become more active, melanin production increases, often leading to a darkening of eye color within the first few months or years of life.
Aging can also bring about subtle changes in eye color. Some individuals may experience a slight darkening or lightening of their eyes over many years due to minor accumulation or loss of melanin. These age-related shifts are very gradual and do not result in dramatic transformations, such as from brown to green.
Certain medical conditions, though rare, can cause changes in eye color. Heterochromia, a condition where irises are different colors or have color variations within a single iris, can be present from birth or acquired later in life due to injury or disease. Conditions like Fuch’s heterochromic iridocyclitis (an inflammatory condition), Horner’s syndrome, or pigment dispersion syndrome may lead to a change in iris pigmentation, often resulting in a lighter or darker appearance in one eye.
Specific medications can also cause iris color changes as a side effect. Prostaglandin analogs, a class of eye drops prescribed for glaucoma, can lead to a permanent darkening of the iris, particularly in individuals with mixed-color eyes like green-brown or blue-brown. This darkening occurs due to increased melanin production in the iris. Finally, temporary factors like mood swings or diet do not alter actual eye color, despite popular beliefs.