The color of a person’s eyes is determined by the amount and type of pigment in the iris of their eyes. The two main pigments that influence eye color are melanin and lipochrome. The amount of these pigments present is controlled by multiple genes.
Melanin
Melanin is the primary pigment that determines eye color. It comes in two forms:
- Eumelanin – A brown/black pigment
- Pheomelanin – A red/yellow pigment
The amount of melanin present in the iris determines how dark a person’s eyes are. Eyes low in melanin are blue, while eyes with more melanin are green, hazel, or brown.
Lipochrome
Lipochrome is a yellow pigment. Along with melanin, the amount of lipochrome present affects the final color of the eyes.
Here is an overview of the main genes involved in eye color determination:
HERC2
The main gene that controls the amount of melanin produced is HERC2. This gene regulates expression of the OCA2 gene. Specific variants of HERC2 reduce the amount of melanin in the eyes, leading to blue/green color.
OCA2
The OCA2 (oculocutaneous albinism II) gene encodes an enzyme involved in melanin production. The HERC2 gene regulates OCA2 activity. Certain mutations in OCA2 result in less melanin pigment in the eyes.
SLC24A4
The SLC24A4 gene helps regulate melanin production and delivery to melanosomes in the iris. Variants of this gene are linked to differences in eye color between brown and blue/green eyes.
TYR
The TYR gene encodes the enzyme tyrosinase, which is important for melanin production. Mutations in TYR can reduce melanin levels and lead to blue eyes.
The Inheritance of Eye Color
Eye color is a polygenic trait, meaning multiple genes influence the phenotype. However, the inheritance of eye color generally follows simple Mendelian genetics:
- Brown eye color (BE) is dominant over blue/green eye color (be).
- Therefore, brown eyes are a dominant trait and blue/green eyes are a recessive trait.
If both parents have brown eyes, all offspring will have brown eyes. If one parent has homozygous blue eyes (be/be) and the other has brown eyes (BE/be), the possible offspring genotypes and phenotypes are:
| Offspring Genotype | Offspring Phenotype |
|---|---|
| BE/be | Brown eyes |
| be/be | Blue eyes |
There is a 50% chance of a brown-eyed parent with the genotype BE/be having a blue-eyed child. However, two blue-eyed parents will always have blue-eyed children.
Exceptions
While brown eye color generally acts as a dominant trait, there are some exceptions to these inheritance patterns:
- Incomplete dominance – The HERC2 gene displays incomplete dominance, meaningheterozygotes have an intermediate eye color (hazel or green).
- Epistasis – Other modifier genes can mask the effects of the major eye color genes through epistasis.
- Polygenic inheritance – Minor genes also influence final eye color, complicating simple Mendelian ratios.
Therefore, model organisms with simplified genetics often do not perfectly reflect the polygenic nature of human eye color.
Prevalence of Eye Colors
Globally, brown eyes are the most common eye color. Blue and green eyes are rare traits and are most prevalent in European populations. Here is a breakdown of the general global distribution:
| Eye Color | Global Prevalence |
|---|---|
| Brown | 70-90% |
| Blue | 8-10% |
| Hazel | 5-10% |
| Green | 2% |
| Gray | 1% |
However, in countries such as Iceland and Finland, up to 90% of the population has blue eyes. In contrast, only about 10% of people have blue eyes in countries such as India, Africa, and China.
United States
In the U.S., the prevalence of eye colors is:
- Brown: 55%
- Blue: 27%
- Hazel: 9%
- Green: 7%
- Other: 2%
Changes in Eye Color With Age
While genetics determine eye color from birth, there can be some natural changes as people age:
- Infant eye color is not always the final color.
- Melanin levels may increase over the first few years of life, making eyes darker.
- The iris can also gradually lose pigmentation, making eyes appear to lighten somewhat.
- The limbal ring around the iris may fade, affecting apparent color.
However, these changes are usually subtle shifts in shade rather than a completely different hue.
Can Eye Color Change With Mood or Health?
There are several myths that eye color can change dramatically with mood, emotion, or health status. However, it is anatomically impossible for the eye to physically change color in response to temporary states like mood or fatigue. Any perceived color change is likely due to:
- Lighting conditions
- A change in clothing, makeup, or background
- The size of the pupil
- Redness of the eyes
- A change in eye moisture
While these factors may subtly influence how the eye color is perceived, the pigmentation and genetics remain unchanged.
Conclusion
In summary, the inheritance of human eye color does generally follow simple dominant and recessive patterns. However, the involvement of multiple genes leads to polygenic inheritance that can make predictions more complex. While brown eyes are dominant over blue, factors like incomplete dominance, epistasis, and variable expressivity mean eye color genetics are not always straightforward.
Environmental factors, aging, and pupil responses can also subtly alter eye color perception. But the fundamental genetic basis of eye pigmentation remains constant over a person’s lifetime. In essence, colored eyes like blue are a recessive trait at the main gene loci, but the final determination of eye shade involves many genetic and developmental factors.
