Eye Color Punnett Square Calculator

This calculator predicts the possible eye colors of a child based on the eye colors of both parents. You select the mother’s and father’s eye color from the drop-down lists, and the calculator returns the probability of each eye color (brown, green, blue, and other) for their offspring. It uses a simplified two-gene model (BEY2 and GEY2) that explains most, but not all, human eye color inheritance.

Formula

The calculator uses a two-gene model for eye color. The first gene (called BEY2 or OCA2) has a dominant allele for brown (B) and a recessive allele for blue (b). The second gene (GEY2 or HERC2) has a dominant allele for green (G) and a recessive allele for blue (g).

Each parent contributes one allele from each gene. The table below shows the possible genotypes and their resulting phenotypes:

• Brown eyes: at least one B allele (BB or Bb) regardless of the G gene.
• Green eyes: no B allele (bb) and at least one G allele (GG or Gg).
• Blue eyes: no B allele and no G allele (bbgg).

For the calculator, we assume each parent has the most common genotype for their eye color: Brown = BbGg (heterozygous for both), Green = bbGg, Blue = bbgg. The probabilities are then derived from a Punnett square of these genotypes.

Worked Example

Suppose both parents have brown eyes (the default). According to our model, each parent is BbGg. We set up a 4×4 Punnett square for the two genes.

First, list all possible gametes from each parent: BG, Bg, bG, bg. The Punnett square has 16 cells. Count the phenotypes:

• Cells with at least one B: 12 out of 16 → 75% brown.
• Cells with bb and at least one G: 3 out of 16 → 18.75% green.
• Cells with bbgg: 1 out of 16 → 6.25% blue.

So the calculator shows Brown: 75%, Green: 18.75%, Blue: 6.25% — exactly as in the output above.

How to Use the Calculator

Select the mother’s and father’s eye color from the drop-down menus. The three options are Brown, Green, and Blue. The calculator instantly displays the probability of each possible child eye color as a percentage. The three probabilities always add up to 100%.

For example, if one parent has brown eyes and the other has blue eyes, the calculator gives Brown: 50%, Green: 0%, Blue: 50%. That means a child of these parents has an equal chance of brown or blue eyes, and essentially no chance of green under this model.

Why It’s a Simplified Model

Real eye color is more complex than the two-gene model. Scientists have identified at least 16 genes that influence eye color, with the OCA2 and HERC2 genes playing the largest roles. The simplified model works well for predicting the most likely outcomes in many families, but it cannot account for:

• Intermediate shades like hazel or amber.
• Cases where two blue-eyed parents have a brown-eyed child (rare, but possible due to other genes).
• Polygenic inheritance where multiple genes each contribute small effects.

Therefore, treat the output as a rough guide, not a guarantee.

Common Mistakes and Misconceptions

A frequent misconception is that eye color follows simple Mendelian dominance (brown dominant, blue recessive). That is true for the main gene, but the green gene adds another layer. Another mistake is assuming that if both parents have brown eyes, the child must have brown eyes. As the calculator shows, two brown-eyed parents can have a blue-eyed child about 6% of the time under this model.

Also, some people think that blue eyes are impossible if any grandparent had brown eyes. That is incorrect because the child inherits genes from the parents only, not directly from grandparents. The calculator correctly ignores grandparental eye color.

Limitations and Accuracy

The calculator’s accuracy depends on the assumption that each parent carries the most common genotype for their phenotype. In reality, a brown-eyed parent could be BB or Bb, and a green-eyed parent could be GG or Gg. The actual probabilities vary depending on the exact genotype. For instance, if a brown-eyed parent is BB (homozygous), then all children will have at least one B allele, so none can be blue-eyed. The calculator gives a 50% chance of blue eyes if the other parent is blue-eyed, but that only holds if the brown-eyed parent is Bb.

Without genetic testing, you cannot know the exact genotype. Thus, the calculator provides an average estimate based on the most likely genotypes in the general population.

Frequently Asked Questions

Can two blue-eyed parents have a brown-eyed child?

Under the two-gene model, no. Two blue-eyed parents (bbgg) can only pass on b and g alleles, so the child must be bbgg, which is blue. However, in reality, due to other genes and rare mutations, it is possible but extremely rare. The calculator correctly shows 0% for brown and green in that case.

Why does the calculator show different probabilities for brown+green vs. green+brown?

It doesn’t. The order of the parents does not matter. The calculator uses the same logic regardless of which parent is listed as mother or father. The probabilities are symmetric, so brown+green gives the same result as green+brown.

What is the most common eye color in the world?

Brown is the most common eye color globally, with an estimated 55-79% of the world’s population having brown eyes. Blue is the second most common, followed by green and other shades. The calculator reflects this by giving brown the highest probability in most combinations.

Can the calculator predict eye color for a specific child?

No, it only gives probabilities. Each child is an independent event, so even if the chance of blue eyes is 25%, it does not mean that if you have four children, one will definitely have blue eyes. It just means that each child has a 25% chance.

Is eye color determined by a single gene?

No. Historically, it was thought to be a single gene, but we now know it involves multiple genes. The two-gene model used here is a simplification that explains the three main colors, but it is not the full story.

Related

• DNA Activity Coefficient Calculator and Formula
• 4×4 Punnett Square Calculator: How It Works
• 2×2 Punnett Square Calculator: Genotype and Trait Odds

sel mother | Mother's eye color | Brown=1, Green=2, Blue=3
sel father | Father's eye color | Brown=1, Green=2, Blue=3
out brown | Probability of Brown eyes | % | (mother==1 && father==1) ? 75 : (mother==1 && father==2) ? 50 : (mother==1 && father==3) ? 50 : (mother==2 && father==1) ? 50 : (mother==2 && father==2) ? 25 : (mother==2 && father==3) ? 0 : (mother==3 && father==1) ? 50 : (mother==3 && father==2) ? 0 : 0 | 0
out green | Probability of Green eyes | % | (mother==1 && father==1) ? 18.75 : (mother==1 && father==2) ? 37.5 : (mother==1 && father==3) ? 0 : (mother==2 && father==1) ? 37.5 : (mother==2 && father==2) ? 50 : (mother==2 && father==3) ? 50 : (mother==3 && father==1) ? 0 : (mother==3 && father==2) ? 50 : 0 | 0
out blue | Probability of Blue eyes | % | (mother==1 && father==1) ? 6.25 : (mother==1 && father==2) ? 12.5 : (mother==1 && father==3) ? 50 : (mother==2 && father==1) ? 12.5 : (mother==2 && father==2) ? 25 : (mother==2 && father==3) ? 50 : (mother==3 && father==1) ? 50 : (mother==3 && father==2) ? 50 : 100 | 0