Where eye colour comes from
The colour of the iris depends mostly on how much melanin, a brown pigment, it holds. Lots of melanin gives brown eyes, a medium amount gives green or hazel, and very little gives blue. Blue eyes contain no blue pigment at all: they look blue for the same reason the sky does, because of how light scatters.
How much melanin the iris makes is set by genes inherited from both biological parents. The biggest effect comes from a region on chromosome 15 that holds two genes, OCA2 and HERC2. Several other genes add smaller effects.
The two-gene model
Long before those genes were found, Danish researchers studying hundreds of families described eye colour with two genes, and this model is still the simplest way to explain it:
- bey2 (chromosome 15, today linked to OCA2 and HERC2) comes in a brown and a blue version. Brown is dominant.
- gey (chromosome 19) comes in a green and a blue version. Green is dominant over blue, but a brown bey2 hides it.
Everyone has two copies of each gene. Any brown copy gives brown eyes. With no brown copy, any green copy gives green or hazel eyes. Only blue on all four copies gives blue or grey eyes.
The assumption about hidden genes
You cannot see whether a brown-eyed parent carries a blue version underneath. The calculator assumes each parent carries one copy of every hidden version their eye colour allows:
- Brown eyes: one brown and one blue bey2, one green and one blue gey.
- Green eyes: two blue bey2, one green and one blue gey.
- Blue eyes: blue on all four copies.
This is the assumption behind the figures most often quoted for eye colour, such as 75% brown for two brown-eyed parents. If a brown-eyed parent really has two brown copies, all their children will have brown eyes, so for some families the chance of lighter eyes is lower than shown.
Every combination at a glance
| Parents | Brown | Green or hazel | Blue or grey |
|---|---|---|---|
| Brown and brown | 75% | 18.75% | 6.25% |
| Brown and green | 50% | 37.5% | 12.5% |
| Brown and blue | 50% | 25% | 25% |
| Green and green | 0% | 75% | 25% |
| Green and blue | 0% | 50% | 50% |
| Blue and blue | 0% | 0% | 100% |
A worked example
Take a brown-eyed parent and a blue-eyed parent. For bey2, the brown-eyed parent passes on brown or blue, half and half, and the blue-eyed parent can only pass on blue. So half of their children get a brown copy and have brown eyes.
The other half have two blue bey2 copies, and gey decides. The brown-eyed parent passes on green or blue, half and half, and the blue-eyed parent passes on blue. So a quarter of all children have green eyes and a quarter have blue eyes. The result: 50% brown, 25% green, 25% blue.
Why your baby's eyes may still change
Many babies, especially those with light skin, are born with blue or grey eyes. The iris keeps adding melanin during the first months, so eyes often darken to green, hazel or brown. Babies born with brown eyes almost always keep them: in a US study that followed babies to age 2, 73 of the 77 born with brown eyes still had brown eyes. Most children have their lasting eye colour by about their first birthday, and small shifts can continue until about age 3.
Limits of the model
The model is simplified: real inheritance is more complex. MedlinePlus Genetics lists OCA2 and HERC2 plus eight other genes that shape eye colour, and their combined effect is more like a dimmer than an on and off switch. That is why two blue-eyed parents occasionally have a brown-eyed child, something the two-gene model calls impossible. Take the percentages as a fun guide to what is likely, not a prediction for your baby.