Turn Your Love Story Into a Glimpse of Tomorrow With Our AI Baby Generator
One-time payment, no subscription
The HERC2 gene controls whether the OCA2 eye color gene is turned on or off in the iris. A single genetic variant in HERC2 called rs12913832 is the most powerful single switch for eye color in humans: when active, eyes become brown; when silenced, eyes become blue. This one variant explains most of the brown-vs-blue eye distinction in people of European ancestry.
HERC2 itself does many other things in the body. Its role as the eye color switch was only discovered in 2008, transforming how scientists understand eye color inheritance.
HERC2 codes for a large protein involved in many cellular processes:
In addition to all this, a small section of the HERC2 gene contains a regulatory element that controls the OCA2 gene located nearby on chromosome 15.
This regulatory section is essentially an "on/off switch" for OCA2. The specific variant in this switch, rs12913832, decides whether OCA2 produces melanin in the iris.
The rs12913832 site in HERC2 is a single nucleotide polymorphism (SNP). At one specific position in the HERC2 gene, the DNA can have either an A or a G nucleotide. This one-letter difference dramatically changes eye color:
rs12913832 genotypeOCA2 expressionTypical eye colorA/A (homozygous A)FullBrownA/G (heterozygous)PartialBrown, hazel, or greenG/G (homozygous G)SuppressedBlue
The G variant is recessive. A baby needs two G copies (G/G) for blue eyes. With one or two A copies, the baby produces enough OCA2 melanin for brown or intermediate eye colors.
This is one of the cleanest examples of how a single DNA letter can have a major visible effect on appearance.
A 2008 study from the University of Copenhagen made a remarkable discovery: nearly all blue-eyed people share the exact same G variant in HERC2, inherited from a common ancestor who lived 6,000 to 10,000 years ago.
Before this ancestor, every human had brown eyes. A single mutation in HERC2 created the blue-eye variant. The variant then spread through the population over millennia, especially in Northern Europe.
This means blue eyes are a relatively recent feature in human history, and all blue-eyed people are very distant cousins through that shared ancestor.
Inheriting eye color through HERC2 follows a recessive pattern for the blue-eye variant:
Both parents have at least one A copy. They may both be heterozygous (A/G) carriers of the blue variant. The Punnett square outcome:
So two brown-eyed carrier parents have a 1 in 4 chance of a blue-eyed baby.
The brown parent might be A/A or A/G. The blue parent is G/G:
Both parents are G/G. All babies inherit G from both parents and are G/G with blue eyes. The probability is essentially 100% (very rare exceptions from other modifier genes).
HERC2 and OCA2 sit close to each other on chromosome 15. The rs12913832 variant in HERC2 is technically inside HERC2's DNA sequence, but it specifically controls the activity of OCA2 in the iris.
The mechanism:
This is one of the most precisely understood gene regulatory interactions in human biology. A single nucleotide controls the activity of a neighboring gene, which then controls a visible trait.
While HERC2 is the master switch, at least 16 other genes influence eye color:
These modifier genes explain why eye color is not strictly binary (brown vs blue). They produce green, hazel, and various shades of brown. They also explain why some individuals have eye colors that do not match what their HERC2 variant alone would predict.
HERC2 is a large gene with many other variants. Some of these:
The vast majority of HERC2 variants do not affect eye color. The rs12913832 SNP is a specific case in an isolated regulatory region.
The blue-eye variant (rs12913832 G allele) is most common in Northern European populations:
This reflects the geographic history of the blue-eye variant spreading from a common ancestor in Northern Europe.
Direct genetic tests like 23andMe report the rs12913832 status, telling you whether you have A/A, A/G, or G/G. Medical genetic testing can also identify other HERC2 variants relevant to specific conditions.
For prediction purposes, knowing your rs12913832 status tells you most of what you need about your eye-color genes. Combined with OCA2 testing, you can predict baby eye color with reasonable accuracy using a Punnett square.
For visual prediction without DNA testing, our baby eye color calculator factors in parent and grandparent eye colors. For overall appearance, the AI baby face generator at PredictMyBaby reads both parents' photos directly.
A baby inherits one HERC2 copy from each parent. The combinations and outcomes:
Baby gets A from both. Genotype A/A. Brown eyes very likely.
Baby gets A from first parent, A or G from second. 50% A/A, 50% A/G. Brown eyes or intermediate.
Baby gets either A or G from each. 25% A/A, 50% A/G, 25% G/G. 75% chance of brown/intermediate, 25% chance of blue.
Baby gets A or G from first, G from second. 50% A/G, 50% G/G. 50% brown/intermediate, 50% blue.
Baby gets G from both. Genotype G/G. Blue eyes very likely.
HERC2 codes for a multi-purpose protein involved in many cellular processes including DNA repair and protein recycling. A specific section of HERC2 also contains the rs12913832 variant, which controls whether the neighboring OCA2 gene is turned on or off in the iris. This single variant is the major determinant of brown vs blue eyes.
rs12913832 is a single nucleotide polymorphism (SNP) inside the HERC2 gene. The DNA at this position can be either A or G. Two A copies produce brown eyes. One A and one G produce brown, hazel, or green. Two G copies produce blue eyes.
Genetically, yes. A 2008 study found that nearly all people with the blue-eye variant of rs12913832 inherited it from a single common ancestor who lived 6,000 to 10,000 years ago in northern Europe. This variant then spread through subsequent generations.
Very rarely. Both blue-eyed parents are G/G at rs12913832. The baby will inherit G from both, producing G/G genotype. The exception comes from other modifier genes that can occasionally override the HERC2 switch, but this is unusual (less than 5% of cases).
HERC2 follows standard chromosome inheritance: the baby gets one copy from each parent. For the rs12913832 variant specifically, blue eyes require two copies of the G variant (one from each parent), making it recessive in inheritance pattern.
The full HERC2 gene affects many cellular processes. The rs12913832 variant specifically affects eye color. Other HERC2 mutations can cause Angelman syndrome and related conditions, but these are unrelated to the eye color switch.
Want to see what color eyes your baby is most likely to have based on your family's eye color history? Try our baby eye color calculator for a free multi-generation prediction. For full appearance visualization, the AI baby face generator at PredictMyBaby creates a realistic prediction from both parents' photos.