Article

aliases: - "Eye Colour" - "Iris Colour" title: Eye Colour

wikipedia: https://en.wikipedia.org/wiki/Eye_color

Eye Colour

Eye colour is a polygenic phenotypic trait determined by the amount and type of melanin in the iris, the structure of the iris stroma, and the scattering of light (Tyndall effect). The most common human eye colours are brown, blue, green, hazel, amber, and grey, with rare cases of red or violet in severe albinism. Eye colour is influenced by at least 16 genes, with OCA2 and HERC2 on chromosome 15 being the most significant.

Biochemistry of Eye Colour

Eye colour is determined by the type and amount of melanin in the iris. Two main types of melanin are present:

  • Eumelanin: Brown/black pigment, dominant in brown eyes.
  • Pheomelanin: Yellow/red pigment, present in green, hazel, and amber eyes. The ratio and distribution of these pigments, along with the structure of the iris stroma, create the full spectrum of human eye colours. The pigment epithelium at the back of the iris is always rich in melanin, even in blue eyes.

Advanced Genetics and Inheritance

Eye colour inheritance is highly polygenic, involving at least 16 genes. The most significant are:

  • OCA2: Central to melanin production; variants cause blue, green, and brown eyes, and are linked to albinism and hypopigmentation. Regulatory SNPs in OCA2 affect expression and thus pigmentation.
  • HERC2: Regulates OCA2; a specific mutation (rs12913832) is strongly linked to blue eyes.
  • SLC24A4 and TYR: Associated with differences between blue and green eyes.
  • SLC45A2, SLC24A5, IRF4, and others: Minor contributors to hue and saturation. Recent studies show that about 50% of eye colour variation can be explained by known genetic loci, with the rest due to complex interactions.

Eye Colour Changes

Eye colour can change in infancy (as melanin accumulates), with age, or due to disease, trauma, or certain medications (e.g., prostaglandin analogues for glaucoma). Heterochromia can be congenital or acquired.

Types of Eye Colour (Expanded)

Types of Eye Colour

  • Brown: Most common worldwide (~79%); high melanin concentration in the iris stroma and pigment epithelium. Found in nearly all populations, especially in Africa, Asia, and the Americas.
  • Blue: Low melanin; blue appearance results from structural colour (Rayleigh scattering) in the stroma. Most common in northern and eastern Europe, but present globally. Blue eyes are highly sexually dimorphic, more common in men.
  • Green: Moderate melanin and yellowish pigment (lipochrome); rarest natural colour after grey. Most common in northern and central Europe, especially among women.
  • Hazel: Combination of brown and green, often with flecks or a multicoloured appearance. Caused by moderate melanin and Rayleigh scattering. Common in Europe and the US.
  • Amber: Solid yellowish/golden or coppery tint, due to lipochrome pigment. Very rare (about 5% globally).
  • Grey: Very rare (about 3% globally); may result from increased collagen in the iris stroma, causing Mie scattering. Found in Europe, the Middle East, and parts of Asia and Africa.
  • Red/Violet: Seen only in severe albinism, where lack of melanin allows blood vessels to show through.

Non-Human Eye Colour Diversity

Most mammals have brown or darkly pigmented irises. Some animals (e.g., cats, dogs, horses) show blue, green, amber, or sectoral heterochromia due to different genetic mechanisms. In birds, reptiles, and fish, eye colour can be highly variable and is often used for species recognition or camouflage.

Genetics and Inheritance

Eye colour inheritance is complex and polygenic. The main genes are:

  • OCA2: Central to melanin production; variants cause blue, green, and brown eyes, and are linked to albinism.
  • HERC2: Regulates OCA2; a specific mutation is strongly linked to blue eyes.
  • SLC24A4 and TYR: Associated with differences between blue and green eyes.

Sex is a major factor: women tend to have darker eyes than men, possibly due to higher oestrogen levels. Eye colour can change in infancy and sometimes with age, especially in those with hazel or green eyes.

Eye Colour and Sex

Sex hormones (notably oestrogen) may influence melanin production, leading to subtle sex differences in eye colour distribution. Some studies suggest blue eyes are more common in men, while women tend to have darker eyes.

Iris Structure and Limbal Ring

The iris is the coloured part of the eye, with the stroma (front layer) and pigment epithelium (back layer). The limbal ring is a dark ring around the iris, most visible in lighter eyes and associated with youth and attractiveness. See Iris and Limbal ring.

Microanatomy

The iris consists of:

  • Anterior limiting layer
  • Stroma (with blood vessels, melanocytes, and connective tissue)
  • Sphincter and dilator muscles (control pupil size)
  • Pigment epithelium (blocks light) The limbal ring is formed by the optical properties of the corneal limbus and is most prominent in healthy, youthful eyes. See Iris and Limbal ring.

Special Cases and Rare Variations

  • Heterochromia: One iris is a different colour from the other, or a segment of one iris differs (see Heterochromia).
  • Aniridia: Congenital absence of the iris, causing extreme light sensitivity (see Aniridia).
  • Ocular albinism: Lack of melanin, leading to very light, pink, or red-appearing eyes (see Ocular albinism).
  • Kayser–Fleischer ring: A golden-brown ring at the corneal margin, sign of Wilson's disease (see Kayser–Fleischer ring).

Other related conditions:

  • Waardenburg syndrome: Genetic disorder causing heterochromia, deafness, and pigmentary anomalies (see Waardenburg syndrome).
  • Pigment dispersion syndrome: Loss of iris pigment, can cause heterochromia (see Pigment dispersion syndrome).

Medical Implications

  • Lighter eye colours (blue, green, grey) are more sensitive to sunlight and have a higher risk for age-related macular degeneration and uveal melanoma.
  • Darker eyes may have a higher risk of cataracts.
  • The limbal ring's prominence may indicate youth and health.
  • Eye colour outside the iris (e.g., yellowing of the sclera) can signal disease (e.g., jaundice).

Eye Colour and Disease

  • Wilson's disease: Copper accumulation causes Kayser–Fleischer rings.
  • Jaundice: Yellowing of the sclera signals liver disease.
  • Uveitis: Grey iris may indicate inflammation.
  • Albinism: Lack of melanin increases light sensitivity and risk of visual impairment.
  • Waardenburg syndrome: Heterochromia, deafness, and facial features.
  • Pigment dispersion syndrome: Can cause secondary glaucoma.

Evolutionary and Cultural Aspects

  • Eye colour diversity is greatest in people of European descent, likely due to ancient migration and sexual selection.
  • Blue eyes may have been adaptive at higher latitudes, increasing intraocular light scattering and possibly reducing depression risk in low-light environments.
  • Eye colour is often associated with personality, beauty standards, and cultural symbolism. The limbal ring is considered attractive and a sign of youth.

Evolutionary Notes

The OCA2 gene variant for blue eyes likely arose in Europe 6,000–10,000 years ago. Eye colour diversity may have been driven by sexual selection and adaptation to varying UV levels. In non-human animals, eye colour often serves for camouflage, mate selection, or species recognition.

Writing Tips

  • Use detailed sensory descriptions: colour, limbal ring, pupil dilation, and emotional cues.

  • Reference physiological changes (e.g., pupil dilation, limbal ring prominence) to convey mood or attraction.

  • Consider cultural and individual variation in eye colour and enhancement practices (e.g., limbal ring contact lenses).

  • Use rare eye colours or conditions (heterochromia, aniridia, ocular albinism) to create memorable characters or signal uniqueness.

  • Use eye colour changes (due to emotion, health, or age) to foreshadow character development or plot twists.

  • Symbolic use: blue eyes for coldness or mystery, brown for warmth or reliability, green for envy or magic, etc.

  • Describe the limbal ring, sclera, and pupil as part of a holistic eye description.

Example

Her eyes were a stormy grey, the limbal ring stark against the pale iris—a gaze that seemed to pierce through shadow and sunlight alike.

Why it works: Combines rare colour, limbal ring, and metaphor to evoke mood and visual impact.

See Also