Remember how your high school biology instructor taught you about dominant and recessive traits using eye color as an example? Well, your teacher was a fraud.
In his/her defense, there have been a number of developments in molecular biology and genetics since it was commonly taught that dark pigmented eyes are a dominant trait and that the lighter blue variants are recessive.
Most observable human traits are polygenic, which means that multiple genes exert influence over the trait in question. Height is a common example, and eye color is another. There are up to 16 different genes that affect eye color. This is why we see such a wide range of variability in the visual appearance of human irises, including different shades of each color, banding, and speckles. It’s been said that your iris is as unique as your fingerprint, and if you pay close attention to the detail of one, you can tell why. Aside from twins, no two people have identical patterns around their irises.
The big two
The major characteristics that influence visible eye color are pigment and connective tissues. The dark furrows emanating from the pupil are a result of a lower density of collagen fibers in the iris, and white specks arise when they’re highly clustered.
Darker eyes, from brown to black, contain higher concentrations of melanocytes that produce the pigment melanin – the same pigment responsible for the darkness of skin. People with lower concentrations of melanin in their irises expose a translucent layer deeper in the eye. When light enters a bright-colored iris, it passes to this translucent layer and is remitted via backscatter as short wavelength light via the Tyndall effect.
The Tyndall effect involves colloids, a special type of solution in which particles remain evenly distributed throughout a solution. Whipped cream and gelatin are examples of colloids, which are differentiated from solutions like salt water or suspensions like salad dressing (which will eventually separate and settle). Human irises are gel-like colloids.
In the late 19th century, John Tyndall devised an equation that explained the light-scattering properties of colloids. When light is shined through a colloid, the density of the solution and wavelengths of the particles affect the wavelength of returning light. This is the same reason why skim milk and motorcycle smoke can sometimes appear blue if placed under intense light. It’s for a similar reason that the sky appears blue.
In regard to human eyes, melanin absorbs the light before it hits the deeper translucent layer, whereas the absence of melanin allows scattering that emits shorter wavelengths of light, resulting in blue color.
In individuals with heterochromia, where each eye appears to be a different color, both eyes have the same pigments. However, one iris produces less melanin than the other as a result of either genetic or environmental factors, and one eye thus scatters light rather than absorbing it.
Decrease in melanocyte production in human irises is linked to a mutation in OCA-2, which codes for a protein critical to their development. DNA investigations from archeological finds show that all European hunter-gatherers from the Mesolithic period show the mutation for OCA-2 that gives rise to light-colored eyes.
What about green eyes?
Both green and blue eyes arise as a result from the mutation in OCA-2. In other words, both sets of eye colors arise as a result of less melanin secretion in the iris. The differences in color arise as a result of underlying structure. Green eyes still have excess collagen buildup in the inner layer of the iris, whereas blue eyes do not. The density of collagen in green irises causes less scattering, leading to a longer wavelength of emitted light.