Baby Eye Color Predictor (Calculator)

How it works

Gene 1 (bey2, HERC2/OCA2 region): brown B dominant over b · Gene 2 (gey): green G dominant over blue g · eye color: any B → brown; bb with any G → green; bb gg → blue · each parent's hidden genotype is inferred with Bayes' rule from their own and their parents' eye colors (equal allele frequencies as the starting assumption), then each parent passes one allele of each gene at random (Punnett square)

Eye color is polygenic, but most of the difference between brown, green and blue eyes comes from a small region on chromosome 15 (HERC2/OCA2), which is why a simple two-gene model is the classic teaching tool. A brown-eyed parent may secretly carry a blue allele; knowing the grandparents' eye colors changes how likely that is, and this calculator works it out with Bayes' rule instead of assuming every brown-eyed parent is a carrier. Treat the percentages as the model's honest best guess, not a guarantee: two blue-eyed parents occasionally have a brown-eyed child, and most babies of European ancestry are born with blue or gray eyes that darken over the first year or two.

Worked example

A brown-eyed mother and a blue-eyed father, grandparents unknown: the model gives the mother a 66.7% chance of carrying a hidden blue allele, so the baby's most likely eye color is Brown — 66.7%, with 16.7% green/hazel and 16.7% blue/gray. If the maternal grandmother had blue eyes, the mother must carry the allele and the odds become 50% brown, 12.5% green/hazel and 37.5% blue/gray.

Sources: MedlinePlus Genetics: Is eye color determined by genetics? · White & Rabago-Smith (2011), Genotype–phenotype associations and human eye color, J Hum Genet

Frequently asked questions