Eye color (blue, green/hazel, brown)

Eye color is mostly set by a HERC2 switch that turns brown pigment up or down, with an OCA2 variant that can nudge a result toward green or hazel. Reading the two together predicts color better than the main switch alone.

How two markers get you most of the way

Eye colour is not a single-gene trait, but it comes closer than almost any other visible human trait. One variant in HERC2 acts as a switch on the neighbouring OCA2 gene, controlling how much brown pigment the iris makes, and it does most of the work in separating blue from brown.

The second marker, in OCA2 itself, shifts results toward green and hazel, which are intermediate states rather than separate colours: they arise when the iris makes some brown pigment but not much.

How well this actually predicts

The HERC2 variant explains on the order of 46% of the quantitative variation in eye colour on a hue and saturation scale, and it distinguishes blue from brown with an area under the curve near 0.88. Forensic panels built around it and a handful of other markers predict blue or brown correctly about 94% of the time in European populations.

The same panels drop to roughly 73% accuracy for intermediate colours, and perform worse in admixed populations. That is precisely the pattern this site's own coverage note describes: blue and brown are reliable, green and hazel are rough.

How common each version is

In 1000 Genomes reference data, about 59% of European-ancestry individuals carry two copies of the blue-associated HERC2 allele and 34% carry one. In East Asian samples that allele is essentially absent, at under 0.1%, and it is uncommon in African-ancestry samples.

That distribution is why eye-colour prediction works far better in European-ancestry populations than elsewhere, and why blue eyes are the textbook example of a recent, geographically concentrated human variant.

What this panel does not cover

At least a dozen other genes shift eye colour, including SLC24A4, TYR and SLC45A2, and none are read here. Iris colour also changes with age in many children, and adult colour can look different under different light because iris pigment scatters as well as absorbs.

Nothing about eye colour carries health information. The one exception worth naming is that it is not a reliable guide to paternity or family relationships, an old classroom exercise that produced a great deal of unnecessary worry: two blue-eyed parents can have a brown-eyed child, because the trait is polygenic.

What each HERC2 + OCA2 result means

Brown eyes likely. Your HERC2 switch keeps brown pigment turned up and you don't carry the OCA2 green-shifter, so brown eyes are very likely. Other genes can still add subtle green or hazel tones.

Brown likely, possible green tint (HERC2 ×0, OCA2 R419Q ≥1). Your HERC2 switch favors brown pigment, but the OCA2 green-shifting variant you carry can lighten that toward hazel or green-brown. Brown is still the most likely base color.

Brown or green likely (HERC2 ×1). Your HERC2 switch is half-on, the intermediate setting, without the OCA2 green-shifter. People with this mix usually have brown, sometimes green or hazel eyes, because the brown-pigment side tends to win out.

Green or hazel likely (HERC2 ×1, OCA2 R419Q ≥1). Your HERC2 switch is in the intermediate setting and you carry the OCA2 green-shifting variant, a combination that often lands on green or hazel. Outcomes in this middle zone are the least certain of all eye colors.

Green or hazel possible (blue base) (HERC2 ×2, OCA2 R419Q ≥1). Your HERC2 switch points toward blue, but you also carry the OCA2 modifier variant that can add green tones. This is the classic 'expected blue, came out green' combination, so green or light hazel is a real possibility alongside blue.

Blue eyes likely (HERC2 ×2, OCA2 R419Q ×0). Your HERC2 switch is in the low-pigment setting that most strongly predicts blue eyes, and you don't carry the OCA2 modifier that pushes toward green. Blue is the most likely outcome, though other pigment genes can still shift the final shade.

Evidence & sources

This panel reads HERC2 (rs12913832) and OCA2 R419Q (rs1800407).

Combines the main HERC2 blue/brown switch with the OCA2 green/hazel modifier. Eye color draws on several more pigment genes not tested here, so green and hazel in particular are only rough predictions.

Common questions

Can two blue-eyed parents have a brown-eyed child? Yes. Eye colour is polygenic, and the simple dominant-recessive model taught in schools is wrong. It is uncommon but entirely possible, and it is not evidence about parentage.

Why is green eye colour hard to predict? Green and hazel are intermediate states produced by partial brown pigment, and they depend on more genes than the blue-brown switch does. Forensic panels are about 94% accurate for blue or brown and around 73% for intermediate colours.

Related

Body mass index · Earwax type / body odor · Endurance / aerobic capacity (PPARGC1A) · Freckling / sun sensitivity · Height · Muscle fiber type (power vs endurance) · Skin pigmentation (light vs dark)