Blue Eye Color: Genetics, Melanin, and the Physics of Light Scattering

Blue eyes result from low melanin concentration in the iris stroma combined with Rayleigh scattering of shorter (blue) wavelengths.

Mechanism

• Iris layers: Anterior border layer (melanocytes), stroma (collagen fibrils), posterior pigmented epithelium (dense melanin — always dark).
• Blue eyes: Sparse melanin in anterior border/stroma → incident light penetrates stroma → collagen scatters blue light (Tyndall/Rayleigh effect) → blue reflection. Longer wavelengths pass through to dark epithelium and are absorbed.
• Brown eyes: Dense anterior melanin absorbs most light → brown reflection.
• Green/hazel: Intermediate melanin + lipofuscin (yellow) → green/gray combinations.

Genetics

• Polygenic; major locus: HERC2/OCA2 region (rs12913832) regulates OCA2 expression (melanosome maturation).
• MC1R, TYR, SLC24A4, TYRP1 contribute modifier effects.
• Not simple Mendelian recessive; prediction models (e.g., IrisPlex) achieve ~90% accuracy.

Clinical Notes

• Blue‑eyed individuals: ↑ risk of uveal melanoma, age‑related macular degeneration (less UV shielding).
• Photophobia more common (less iris pigment).
• Waardenburg syndrome: heterochromia, dystopia canthorum, deafness — PAX3/MITF mutations.

Eye color is a structural color—like the blue sky—created by light physics, not blue pigment.