How Do Eye Colors Work?
A person’s eye color depends on how much of a pigment called melanin is stored in the front layers of the iris, the structure surrounding the pupil. Specialized cells called melanocytes produce the melanin, which is stored in intracellular compartment called melanosomes. People have roughly the same number of melanocytes, but the amount of melanin within melanosomes and the number of melanosomes within melanocytes both vary.
Eye color ranges depending on how much melanin is stored in these compartments. In people with blue eyes a minimal amount of melanin is found within a small number of melanosomes. People with green eyes have a moderate amount of melanin and moderate number of melanosomes, while people with brown eyes have high amount of melanin stored within many melanosomes.
Genes involved in determining eye Color
The amount of melanin stored is determined by genes that are involved in the production, transport and storage of melanin.
To date, researchers have discovered more than 150 genes that influence eye color, a number of which have been discovered through studies of genetic disorders. Others have been identified during genomic studies of mice and fish.
Some genes play a major role in determining eye color, while others only have a small contribution.
One region of chromosome 15 contains two genes located near to each other that play major roles in determining eye color. One gene, called OCA2, codes for a protein called P protein, which is involved in melanosome maturation and affects the amount and quality of melanin stored in the iris. A number of genetic variations (polymorphisms) in this gene reduce how much P protein is produced and result in a lighter eye color.
The other main gene involved is called HERC2. Intron 86 on this gene controls the expression of OCA2, activating it or deactivating it as required. At least one polymorphism in this intron reduces the expression and activity of OCA2,which reduces how much P protein is produced.
A number of other genes play smaller roles in eye color. The roles of the genes ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1 are thought to combine with those of OCA2 and HERC2.
Eye color inheritance pattern
Due to the number of genes involved in eye color, the inheritance pattern is complex. Although a child’s eye color can generally be predicted by looking at the color of the parents’ eyes, the polymorphisms that can arise mean a child may well have an unexpected eye color.
A child’s eye color depends on the pairing of genes passed on from each parent, which is thought to involve at least three gene pairs. The two main gene pairs geneticists have focused on are EYCL1 (also called the gey gene) and EYCL3 (also called the bey2 gene).
The different variants of genes are referred to as alleles. The gey gene has one allele that gives rise to green eyes and one allele that gives rise to blue eyes. The bey2 gene has one allele for brown eyes and one for blue eyes. The allele for brown eyes is the most dominant allele and is always dominant over the other two alleles and the allele for green eyes is always dominant over the allele for blue eyes, which is always recessive. This means parents who happen to have the same eye color can still produce a different eye color in their child.
For example, if two parents with brown eyes each passed on a pair of blue alleles to their offspring, then the child would be born with blue eyes. However, if one of the parents passed on a green allele, then the child would have green eyes and if a brown allele was present, then the child would have brown eyes irrespective of what the other three alleles were.
Global Distribution of Eye Colors
While living in English-speaking countries might suggest a balanced mix of blue, hazel, and brown eyes, global statistics paint a different picture. Brown eyes dominate worldwide, making up 70-79% of the population. Interestingly, scientists believe all blue-eyed individuals share a common ancestor who had a specific mutation affecting eye color. Here’s a breakdown of eye color prevalence:
- Brown eyes: 70-79%
- Blue eyes: 8-10%
- Hazel eyes: 5%
- Gray eyes: 3%
- Green eyes: 2%
- Heterochromia: 1%
- Red or violet eyes: Under 1%
How Melanin Determines Eye Color
The iris contains two types of melanin: eumelanin, which produces brown, and pheomelanin, which results in amber, green, and hazel shades. Blue eyes occur with minimal melanin, reflecting light in a way similar to the sky or ocean, rather than containing actual blue pigment. Green eyes result from a delicate balance of limited melanin and a yellowish pigment, enhancing the Tyndall effect — the same phenomenon that makes the sky appear blue. Hazel eyes have enough melanin to override this effect. In cases of albinism, the lack of melanin allows the red of blood vessels to show through, sometimes giving eyes a reddish or violet hue.
The Complex Genetics of Eye Color
Eye color is influenced by up to 16 different genes, making predictions based on parent's eye colors not always straightforward. Small changes in these genes can result in a wide spectrum of possible outcomes. It's common for Caucasian babies to be born with blue or gray eyes that change color as they age, much like hair color can evolve from platinum blonde in childhood to darker shades in adulthood. This change is due to the gradual increase in melanin production, sometimes spurred by light exposure.
Variability and Changes in Eye Color
Many people claim their eye color changes with their environment or mood, but this is usually an illusion caused by lighting, pupil dilation, or the colors they wear. However, eye color can change over time due to aging, and injuries can alter it permanently — one of the most famous examples being the late musician David Bowie.
Artificial Changes in Eye Color
Today, those curious about different eye colors can experiment with colored contact lenses. This can be a fun way to see yourself in a new light, but keep in mind that contacts won't replicate the natural look of the iris's complex surface completely.