Genetics of Eye Colour


Human eye colour is a polygenic trait with a heritability of close to 100%, with seven leading factors explaining three‐quarters of the genetic variance: OCA2, TYR, TYRP1, IRF4, SLC45A2, SLC24A5 and SLC24A4. Many of these genes were already known to be mutated in oculocutaneous albinism, and all are involved in melanin biosynthesis, coding for key enzymes, transcription factors controlling expression of key enzymes or transporters that maintain the acidity of the melanosome interior. At least two of the variants of largest effect abrogate regulatory elements in the associated gene, causing changes in expression level. The differences in eye colour between different populations suggest strong selection, either on correlated traits such as skin colour or perhaps directly.

Key Concepts

  • Variation in eye colour is polygenic, with seven major genes explaining 75% of European population variance.
  • Blue eye colour is strongly predicted by a single nucleotide polymorphism near the OCA2 gene – this is the classical recessive gene inferred from family data. Intermediate eye colours such as hazels and greens are less well predicted.
  • Two of the major loci are in non‐coding regulatory regions, a pattern common to trait loci for other complex traits.
  • The large effect sizes make this an excellent model for the investigation of the architecture of gene action, especially gene‐by‐gene interactions (epistasis), as well as multi‐trait action (pleiotropy).
  • Iris pigmentation loci generally exhibit genomic evidence of strong Darwinian selection, and this may not be completely explained by pleiotropic effects on skin colour.

Keywords: iris colour; Darwinian selection; tyrosinase; oculocutaneous albinism; melanin; TYRP1 ; TYRP2 ; OCA2 ; IRF4 ; SLC45A2

Figure 1. Eye colour rating scale of (Martin ; Schultz ), modified from that of Martin. This was realised as a standardised series of glass eyes set in a handy foldout case.
Figure 2. Proportion of brown‐eyed individuals in samples from different European populations.
Figure 3. Eumelanin biosynthetic pathways.
Figure 4. Cartoon of iris structure.


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Further Reading

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Duffy, David(Mar 2015) Genetics of Eye Colour. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0024646]