About

Eye color inheritance follows a polygenic model, but the dominant contribution comes from two loci: OCA2/HERC2 on chromosome 15 and GEY on chromosome 19. The OCA2 locus governs melanin deposition in the iris stroma. A single copy of the dominant allele B produces enough melanin for brown pigmentation. Two recessive copies (bb) yield low melanin, delegating color to the GEY locus: dominant G produces green or hazel via Rayleigh scattering plus lipochrome, while homozygous recessive gg produces blue. This calculator applies Bayesian inference over parental genotype distributions conditioned on observed phenotypes. Grandparent data narrows carrier probabilities. Without it, population-level Hardy-Weinberg priors are used. Results are statistical estimates. Real-world iris color is influenced by at least 16 genes and epigenetic factors this model does not capture. Expect accuracy within ±10% for the three primary phenotype categories.

Formulas

The two-locus model uses the bey2 locus (alleles B, b) and the gey locus (alleles G, g). Phenotype is determined by the following dominance hierarchy:

{

BROWN if bey2 ∈ {BB, Bb}GREEN if bey2 = bb ∧ gey ∈ {GG, Gg}BLUE if bey2 = bb ∧ gey = gg

The probability of offspring phenotype Ph given parent phenotypes Ph₁ and Ph₂ is computed by marginalizing over all compatible genotype pairs:

P(Ph_child) = ∑i,j P(G₁ = i | Ph₁) ⋅ P(G₂ = j | Ph₂) ⋅ P(Ph_child | i × j)

Where G₁, G₂ are parental genotypes at both loci, i, j index over all compatible genotypes, and the cross probability is computed via a standard Punnett square. Grandparent phenotypes refine the conditional P(G | Ph) using Bayesian updating. For instance, a brown-eyed parent with a blue-eyed grandparent must carry at least one b allele, so P(BB) = 0.

Reference Data

Parent Phenotype Combination	Brown	Green/Hazel	Blue
Brown × Brown	75%	18.75%	6.25%
Brown × Green	50%	37.5%	12.5%
Brown × Blue	50%	25%	25%
Green × Green	0%	75%	25%
Green × Blue	0%	50%	50%
Blue × Blue	0%	1%	99%
Brown (carrier) × Brown (carrier)	56.25%	18.75%	25%
Brown (carrier) × Blue	25%	25%	50%
Brown (non-carrier) × Blue	100%	0%	0%
Brown (non-carrier) × Green	100%	0%	0%
Hazel × Blue	0%	50%	50%
Hazel × Hazel	0%	75%	25%
Hazel × Green	0%	75%	25%
Brown × Hazel	50%	37.5%	12.5%

Frequently Asked Questions

The two-gene model (OCA2/HERC2 and GEY) accounts for approximately 75-85% of eye color variation in European populations. However, at least 16 genes contribute to iris pigmentation. This model will correctly predict the dominant color category (brown, green/hazel, blue) in most cases, but cannot distinguish subtle shades like amber vs. light brown, or gray vs. blue. Accuracy improves significantly when grandparent data is provided.

Under the strict two-gene model, two blue-eyed parents (genotype bbgg) cannot produce a brown-eyed child because neither carries a B allele. In practice, rare epistatic interactions, mutations, or contributions from other pigmentation genes (e.g., SLC24A4, TYR) can produce exceptions. The estimated probability is below 1%.

Grandparent data constrains the possible genotypes a parent can carry. A brown-eyed individual could be BB or Bb at the bey2 locus. If one of their parents had blue eyes (bb), the brown-eyed individual must be Bb (heterozygous carrier). This elimination of BB from the distribution substantially increases the probability of the child inheriting bb and having lighter eyes.

Hazel is modeled as a variant of the green phenotype category. Both arise from the bb genotype at bey2 with at least one G allele at gey. The distinction between green and hazel depends on melanin concentration in the anterior iris stroma, which this model groups together. When a user selects hazel, it is treated genetically equivalent to green for computation purposes.

The Hardy-Weinberg prior allele frequencies used in this calculator are calibrated for populations of European descent, where eye color variation is highest. Populations with predominantly brown eyes (East Asian, African, South Asian) have near-fixation of the B allele at the OCA2 locus. The calculator remains valid for these populations but will predict brown with very high probability, which matches empirical data. Cross-ancestry couples may see intermediate results.

Most infants of European descent are born with blue or gray-blue eyes due to low melanin at birth. Melanocytes in the iris begin producing melanin in response to light exposure during the first 6 to 9 months. Permanent eye color typically stabilizes between 9 and 12 months, though subtle changes can continue until age 3. Infants born with dark brown eyes (common in non-European populations) rarely change color.