What this calculator does
The classic school model treats eye colour as one gene with brown dominant over green and green over blue. That model is genuinely useful for understanding dominance and genuinely wrong about eye colour, which involves around sixteen genes.
Taken on its own terms, it produces sensible-looking odds. Two blue-eyed parents give blue at 100 per cent under the model, and a brown-eyed parent with a blue-eyed one gives three possible outcomes with brown most likely at 33.3 per cent.
The formula
Each parent is assigned the set of alleles their eye colour permits, a Punnett square is computed, and the dominant allele in each pairing determines the child's colour.
| Term | Meaning |
|---|---|
| Dominance | Brown over green, green over blue, in this simplified model. |
| Polygenic | Controlled by many genes, which is what eye colour actually is. |
| OCA2 and HERC2 | The two genes with the largest single effect, though far from the only ones involved. |
The inputs explained
| Field | What to enter |
|---|---|
| Parent 1 eye color | First parent's eye colour. |
| Parent 2 eye color | Second parent's eye colour. |
When to use it
Learning about dominance
Eye colour is the standard teaching example, even though it is a poor one.
Satisfying curiosity
A rough sense of the possibilities, with the caveat that the model is simplified.
Understanding why predictions fail
The gap between the model and reality is itself instructive.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What can a brown-eyed parent produce?
The same parent paired with three different partners.
| Parent 2 eye colour | Most likely eye color | Possible outcomes |
|---|---|---|
| Brown | Brown (55.6%) | 3 |
| Green | Green (50.0%) | 3 |
| Blue | Brown (33.3%) | 3 |
Questions
Can two blue-eyed parents have a brown-eyed child?
The simple model says no, but it does happen, which is one of the clearest demonstrations that the model is incomplete. Multiple genes contribute, and rare combinations can produce brown eyes from two blue-eyed parents.
How many genes actually affect eye colour?
Around sixteen have been identified, with OCA2 and HERC2 carrying the largest effect. That is why the outcome is a continuous spectrum of shades rather than three discrete categories.
Why do babies' eyes change colour?
Because melanin production in the iris continues after birth. Many babies, particularly those of European ancestry, are born with blue or grey eyes that darken over the first year or two.
Should this be relied on?
Not for anything that matters. It is a simplified teaching model, and its predictions are frequently wrong. It says nothing about parentage and should never be treated as if it does.
For blood type inheritance, which the same Punnett logic handles far better, see the blood type calculator. For predicted height, see the mid-parental height calculator.