Blue eyes are not dominant. The idea that eye color is a simple case of a single dominant or recessive trait is a widespread misconception that modern genetic science has shown to be inaccurate. Eye color inheritance is far more intricate, involving the combined influence of multiple genes that work together to determine the final shade of the iris. Understanding eye color requires looking beyond simple Mendelian inheritance models and exploring the complex interplay of pigment production and light physics.
The Outdated Single-Gene Model
For many years, the inheritance of eye color was simplified into a straightforward Mendelian model, which posited that the brown eye color allele was completely dominant over the blue eye color allele. This traditional model suggested that a person needed to inherit two copies of the blue-eye gene—one from each parent—to have blue eyes, while inheriting just one brown-eye gene would result in brown eyes. The logical conclusion of this idea was that two blue-eyed parents could never have a brown-eyed child.
This single-gene concept was widely taught in introductory science classes, providing a basic framework for understanding trait inheritance. However, this model cannot account for the full spectrum of eye colors that exist, such as green, hazel, or gray, or the unexpected color combinations that occasionally appear in families. Eye color is a polygenic trait, meaning it is controlled not by one gene, but by the cumulative effects of many genes working in concert.
The Key Genes That Determine Eye Color
Eye color is primarily determined by the amount and type of melanin pigment present in the iris. While up to 16 different genes may play a role, two genes on chromosome 15—OCA2 and HERC2—are the major players responsible for most of the blue-brown color variation. The OCA2 gene contains the instructions for producing the P protein, which is involved in the creation and storage of melanin in cellular structures called melanosomes.
The activity of the OCA2 gene is heavily influenced by the neighboring HERC2 gene. A specific variation in the HERC2 gene acts like a genetic switch, effectively reducing the expression of OCA2 and limiting the production of the P protein. Less P protein means less melanin is deposited in the iris, resulting in lighter eye colors like blue and green. Conversely, a high concentration of melanin results in darker eyes.
How Blue Eyes Get Their Color
The blue color of the iris is not caused by a blue pigment, which is a common misconception, but rather by a physical phenomenon involving light. All human eyes contain melanin, a brown-black pigment, but blue eyes have very low concentrations of this pigment in the front layer of the iris, known as the stroma. The dark layer of pigment on the back of the iris absorbs most of the longer wavelengths of light, like red and yellow.
When light enters the eye, it encounters the collagen fibers and other particles within the relatively clear stroma. Because there is little melanin to absorb the light in this layer, the shorter, blue wavelengths are scattered back out into the environment, a process known as Rayleigh scattering. This is the same mechanism that makes the sky appear blue. The resulting blue appearance is a structural color that depends on the lighting conditions, which can make the eye appear to change color slightly.
Why Predicting Eye Color is Complex
The involvement of multiple genes makes predicting a child’s eye color more complicated than using the simple Punnett square models from the past. While the OCA2 and HERC2 genes account for a large portion of the eye color variation, numerous other modifier genes contribute smaller, yet measurable, effects. These minor genes can subtly adjust the amount or distribution of melanin, resulting in the continuous spectrum of colors from light blue to dark brown.
The influence of these modifier genes explains why two parents with blue eyes can sometimes produce a child with non-blue eyes, such as green or even light brown. In these rare instances, the combination of multiple minor genes inherited by the child results in a cumulative effect that boosts the melanin production beyond what either blue-eyed parent alone could produce. While a child’s eye color can often be predicted based on the parents’ colors, the multi-gene nature of the trait means that genetic variations can produce unexpected results.