Eye color is determined primarily by genetics, and in most cases, a person’s eye color remains the same throughout their life. However, there are some rare instances where a person’s eye color may appear to change or become multifaceted over time. This is usually not a true change in eye color, but rather a change in color intensity or acquisition of a new color pattern. In very rare cases, true changes in eye color can occur later in life due to various factors.
What Determines Eye Color?
Human eye color is primarily determined by the amount and type of melanin pigment in the iris. Melanin is produced by specialized cells called melanocytes. There are two types of melanin:
- Eumelanin: A brown/black pigment
- Pheomelanin: A red/yellow pigment
The specific combination of these two melanins determines eye color. Brown eyes contain more eumelanin, while green, blue, and hazel eyes contain varying mixtures of eumelanin and pheomelanin. Gray eyes contain very little of either pigment.
Eye color is inherited through several genes, including:
- HERC2 – regulates OCA2 expression
- OCA2 – encodes P protein, involved in melanin production
- SLC24A4 – regulates calcium influx into melanocytes
- SLC45A2 – helps mediate melanin synthesis
- TYR – encodes tyrosinase, an enzyme that catalyzes melanin production
The specific variations (alleles) of these genes present determine the pigmentation and ultimate eye color. While multiple genes influence eye color, HERC2 and OCA2 play the most significant roles.
Can Brown Eyes Turn Green?
Because eye color genetics are complex but generally remain constant throughout life, true changes from brown eyes to green are extremely rare. There are, however, some possible explanations for why brown eyes may appear to turn greenish later in life:
1. Rayleigh Scattering
As people age, the melanin pigment in the iris can start to degrade and clump together. This reduces the overall amount of melanin and allows more light to reflect and scatter off the stroma – the connective tissue beneath the iris. This process, known as Rayleigh scattering, can cause the eyes to take on a more light-reflective appearance. While this can give darker eyes a slightly lighter or hazier appearance, it would not result in a vivid change from brown to green on its own.
2. Fuchs Heterochromic Iridocyclitis
This rare condition involves asymmetric inflammation of the iris that can lead to hypomelanosis – a localized loss of pigmentation in one eye. If it occurs in the brown eye, it can uncover underlying green areas. However, the onset is usually sudden rather than gradual over time. Most cases occur in the blue or green eye instead, turning it a paler blue-grey color.
3. Horner’s Syndrome
Horner’s syndrome involves damage to the sympathetic nervous system, which can disrupt neural signals for pigmentation in one eye. In rare cases, this can lead to the appearance of sectoral heterochromia – different colors within the same iris. If brown irises are affected, small green portions may become visible. However, the onset is usually abrupt rather than gradual.
4. Pigment Dispersion Syndrome
This condition causes pigment from the iris to flake off and disperse into other areas of the eye. It can occur bilaterally or unilaterally, potentially exposing different colors within a brown iris, including green. Gradual onset over years is possible. However, it primarily affects younger adults.
5. Melanoma of the Iris
Melanomas involving the iris are very rare. But if they cause enough widespread loss of melanin, they may potentially unmask underlying green coloration if present. Onset would be more abrupt, however.
6. Albinism
Albinism is a genetic condition that results in little to no melanin pigmentation. While full albinism would make the eyes appear very light blue or reddish, partial or incomplete forms of albinism could potentially unmask underlying green hues in brown eyes over time.
7. Birth Trauma/Sectoral Heterochromia
In very rare cases, physical trauma to the eye shortly after birth can potentially lead to disruption of melanocyte development. This can result in sectoral heterochromia – different colors within the same iris. Trauma could unmask subtle green portions in a predominantly brown iris.
8. Eye Injury/Disease
Physical trauma or inflammation from eye injuries and certain diseases can, in some cases, cause heterochromia or loss of pigmentation in the eye. However, this tends to be sudden rather than gradual and is very rare.
9. Exposure to Industrial Chemicals
There are extremely rare reports of industrial chemical exposure causing eye injury that results in depigmentation months or years later. But evidence is scarce, and any color change would likely be abrupt.
True Changes vs Perceived Changes
While the above explanations provide some plausible mechanisms for brown eyes revealing underlying green hues later in life, these cases are exceptionally rare. Far more commonly, changes in eye color intensity are perceived over time, rather than true changes in color. Reasons for this include:
- Lighting conditions
- Use of color contacts
- Lasik surgery
- Reduction in limbal ring pigment with age
- Change in hair or skin color affecting perceived eye color
- Gradual fading of melanin related to aging
- Observation bias – mistaken recall of original eye color
Subtle changes in color intensity can also develop in adolescence or early adulthood due to hormonal shifts during puberty. But again, these are due to changes in melanin concentration rather than new color acquisition.
The Role of Genetics
While the genetics of eye color are complex, there are two primary reasons why significant color changes from brown to green later in life are highly improbable:
1. Lack of Green-Blue Genetic Variants
For green coloration to manifest, a person must have genetic variants associated with both green and blue eye colors. Brown-eyed individuals lack the specific gene combinations required to produce green pigmentation. Even if melanin concentration decreases with age, this would not unveil green color if the genetic potential for it is not already present.
2. Fixed Melanocyte Populations
During the first year of life, melanocyte cellular density in the iris becomes relatively fixed. While melanin production may decrease over time, the fundamental melanocyte population remains stable. Even with melamin loss, the genetic regulation of melanocytes established in infancy generally persists lifelong.
Conclusion
In summary, while small changes in eye color intensity and appearance are possible later in life, a dramatic switch from brown eyes to vivid green eyes is highly improbable. The only plausible scenarios involve unmasking small sectors of underlying green color via localized pigment loss. But such cases are exceptionally rare and require very specific circumstances. For the vast majority of people, eye color remains fixed from infancy onward according to genetic programing established early in development. So while lightening of dark eyes may occur with age, a complete color change does not take place without the presence of unique genetic factors.