rs12913832: The SNP That Determines Your Eye Color
Learn how the rs12913832 variant in the HERC2/OCA2 gene region determines eye color, what your genotype means, and how allele frequencies differ across populations.
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What Is rs12913832?
rs12913832 is a single nucleotide polymorphism (SNP) located in intron 86 of the HERC2 gene on chromosome 15 [1]. Despite sitting within HERC2, this variant acts as a long-range regulatory element that controls expression of the neighboring OCA2 gene - the major gene responsible for melanin production in the iris [2].
This single SNP is the strongest genetic predictor of eye color ever identified. A landmark 2008 study found that rs12913832 alone explains over 74% of eye color variation in European populations [1]. No other human trait is so strongly influenced by a single genetic variant.
The Two Alleles: A and G
rs12913832 has two alleles:
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Get started- A allele (ancestral): Associated with brown eyes. This allele maintains normal OCA2 expression, leading to higher melanin production in the iris.
- G allele (derived): Associated with blue eyes. This allele disrupts a conserved transcription factor binding site in the HERC2 enhancer, reducing OCA2 expression and melanin levels [2].
What Your Genotype Means
| Genotype | Likely Eye Color | Mechanism |
|----------|------------------|--------------------------------------------------|
| AA | Brown | Full OCA2 expression → high melanin |
| AG | Green/Hazel | Intermediate OCA2 expression → moderate melanin |
| GG | Blue | Reduced OCA2 expression → low melanin |
The A allele is incompletely dominant over G. Heterozygous (AG) individuals typically have intermediate phenotypes - green, hazel, or light brown eyes - though this varies based on other modifying loci [3].
Important caveat: While rs12913832 is the strongest predictor, eye color is not strictly Mendelian. At least 16 genes contribute to iris pigmentation, including SLC24A4, TYR, IRF4, and SLC45A2 [4]. Two blue-eyed parents can occasionally have a brown-eyed child, and vice versa.
Allele Frequencies Across Populations
The frequency of the blue-eye-associated G allele varies dramatically across global populations, reflecting evolutionary selection pressures and migration history [5]:
| Population | A Allele Frequency | G Allele Frequency |
|--------------|--------------------|--------------------|
| Europeans | ~45% | ~55% |
| East Asians | ~99% | ~1% |
| Africans | ~99% | ~1% |
| South Asians | ~90% | ~10% |
The G allele reached high frequencies in Northern Europe through positive selection - possibly driven by sexual selection, vitamin D synthesis advantages in low-UV environments, or both [6]. In populations with African and East Asian ancestry, the ancestral A allele remains nearly fixed, and brown eyes are the predominant phenotype.
European Sub-Populations
Within Europe, there is a clear north-south gradient [5]:
- Scandinavian countries: G allele frequency reaches 70–80%
- Southern Europe (Italy, Greece, Spain): G allele frequency drops to 20–30%
- British Isles/Netherlands: Approximately 50–60%
This gradient correlates with the historical migration of Yamnaya steppe populations and earlier Mesolithic hunter-gatherers who carried the G allele at high frequencies [7].
The HERC2-OCA2 Regulatory Mechanism
The molecular mechanism is well characterized. rs12913832 sits within a highly conserved regulatory element in HERC2 intron 86. The A allele contains a binding site for the transcription factor MITF (melanogenesis-associated transcription factor) and LEF1 [2].
When the G allele is present:
- The MITF/LEF1 binding site is disrupted
- Enhancer activity at the OCA2 promoter decreases
- OCA2 protein (P protein) production drops
- Less melanin is transported to melanosomes in iris melanocytes
- The iris appears blue due to Rayleigh scattering of light through low-melanin stroma [8]
This is a cis-regulatory effect - the HERC2 enhancer physically contacts the OCA2 promoter via chromatin looping across approximately 21 kilobases [2].
Clinical Relevance
Beyond cosmetic curiosity, rs12913832 has several clinical associations:
Forensic DNA Phenotyping
rs12913832 is the cornerstone of forensic eye color prediction systems like IrisPlex [9]. Law enforcement agencies use this SNP (alongside five other markers) to predict eye color from crime scene DNA with approximately 90% accuracy for blue and brown eyes.
Melanoma Risk
The G allele (blue eyes) is associated with a modest increase in cutaneous melanoma risk, consistent with the broader association between light pigmentation and UV sensitivity [10]. Individuals with GG genotype should be particularly vigilant about sun protection.
Age-Related Macular Degeneration
Some studies suggest lighter iris color (GG genotype) may be associated with slightly elevated risk of age-related macular degeneration, though evidence is mixed [11].
How to Check Your Genotype
If you've had DNA testing through 23andMe, AncestryDNA, or another direct-to-consumer service, rs12913832 is included in the standard genotyping arrays. You can find it in your raw data file.
With Genome Insight, you can upload your raw DNA data and instantly see your rs12913832 genotype along with hundreds of other trait, health, and pharmacogenomic variants - all analyzed in seconds with clear, research-backed explanations.
Explore Your Own Genetics
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References
[1] Sturm, R.A. et al. (2008). A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color. American Journal of Human Genetics, 82(2), 424-431.
[2] Visser, M. et al. (2012). HERC2 rs12913832 modulates human pigmentation by attenuating chromatin-loop formation between a long-range enhancer and the OCA2 promoter. Genome Research, 22(3), 446-455.
[3] Liu, F. et al. (2010). Digital quantification of human eye color highlights genetic association of three new loci. PLoS Genetics, 6(5), e1000934.
[4] White, D. & Rabago-Smith, M. (2011). Genotype-phenotype associations and human eye color. Journal of Human Genetics, 56(1), 5-7.
[5] 1000 Genomes Project Consortium. (2015). A global reference for human genetic variation. Nature, 526(7571), 68-74.
[6] Wilde, S. et al. (2014). Direct evidence for positive selection of skin, hair, and eye pigmentation in Europeans during the last 5,000 y. PNAS, 111(13), 4832-4837.
[7] Mathieson, I. et al. (2015). Genome-wide patterns of selection in 230 ancient Eurasians. Nature, 528(7583), 499-503.
[8] Prota, G. (1997). Pigment cell research: what directions? Pigment Cell Research, 10(1-2), 5-11.
[9] Walsh, S. et al. (2011). IrisPlex: A sensitive DNA tool for accurate prediction of blue and brown eye colour. Forensic Science International: Genetics, 5(3), 170-180.
[10] Gudbjartsson, D.F. et al. (2008). ASIP and TYR pigmentation variants associate with cutaneous melanoma and basal cell carcinoma. Nature Genetics, 40(7), 886-891.
[11] Liew, S.H.M. et al. (2006). The role of heredity in determining central corneal thickness and anterior chamber depth. British Journal of Ophthalmology, 90(3), 306-310.
Related Reading
Check Your Own Variants
If you have raw DNA data from 23andMe, AncestryDNA, or similar services, you can analyze the genetic variants discussed in this article. GenomeInsight processes everything in your browser — your data never leaves your device.
Henry Martinez
Genetic health insights for everyone.