Human eye color is determined by pigmentation of the iris. The iris is a muscular connective tissue that contains a small hole in its center (the pupil) that allows light to enter the eye. The constriction and dilation of the iris allow more or less light through the pupil into the eye, depending on the conditions of the environment. Aside from this muscular tissue, the iris also contains cells called melanocytes that contain a pigment called melanin. The amount, type, packaging and quality of melanin in the iris play a large part in determining the color of a person’s eye.
Irises that appear brown contain a lot of melanin, while irises that appear blue contain less melanin. Irises that appear green or hazel contain an intermediate amount of melanin. Melanin is a pigment found in hair, skin as well as the iris. As a pigment, melanin absorbs light, which explains why irises containing more melanin appear darker in color (because the incoming light is absorbed and not reflected). In an iris with less melanin, less light is absorbed, and the iris appears lighter in color. Irises containing less melanin can range in color, including blue, green, gray, and hazel. The specific color that we observe is determined by the tissue characteristics of the iris, which affect light scattering. A green colored iris is produced when incoming light is partially absorbed by melanin and the remaining light is scattered by collagen proteins within the iris tissue to reflect green light. Irises that contain no pigment, found in people with albinism, appear red due to light reflected from blood vessels in the eye.
Human eye color was long thought to be governed by a simply dominant-recessive relationship determined by Mendelian genetics whereby brown eye color was dominant to blue or green/hazel eye color and green/hazel color was dominant to blue. This theory was put forward in the early 1900s and has since been revised due to both pedigree (family history) and genetic mapping studies. If blue eyes were recessive, as originally proposed, then blue-eyed parents could only give birth to children with blue eyes. This is not always the case, however, suggesting to scientists that eye color may be more complicated than originally thought. Based on what we now know, 74% of the variation in eye color is due to mutations in two genes: OCA2 and HERC2. OCA2 is involved in melanin transportation and maturation. The HERC2 gene, though not directly involved in melanin production or maturation, affects the regulation of OCA2. At least 15 additional genes have been identified that also influence eye color. Within these genes, numerous mutations have been linked to predicting blue versus brown eye color, and several mutations have been linked specifically to green or hazel eye color. Aside from understanding eye color, scientists are interested in identifying mutations that are predictive of eye color because of its application in forensics. A suspect commonly leaves DNA at the scene of a crime, and being able to identify a suspect’s eye, hair and skin color may go a long way in identifying the criminal.
Now that we have an understanding of how eye color works, how rare are green eyes? Determining the prevalence of any given eye color is complicated, as eye color is a continuous spectrum from dark brown to light blue. Depending on the scale being used, the proportion of people with a given color eye can be very different. In fact, scientists use several different scales of eye color ranging from two options (brown versus blue) to hundreds of options. To complicate matters further, most worldwide studies combine green and hazel eyes into the same category. Other studies simply report whether participants had brown or blue eyes, and call every thing else “other” or “undefined”, which could include green, hazel, gray, red, amber, mixed, and anything in between. With such variability in how eye color is determined and reported, it is difficult to know how prevalent a given eye color is, especially green.
Several sources claim that worldwide only 1–2% of people have true green eyes; however, it is unclear where this statistic originates. Eye color is very different between different populations and ethnicities. Over 90% of people of African or Asian ancestry have brown eyes, and they account for most of the world’s population. Blue, green and hazel eyes are typically only observed in individuals of European ancestry. Even across Europe the prevalence of eye color varies dramatically. The prevalence of blue or green eyes in European countries ranges drastically, with almost 90% of people having blue or green eyes in some northern European countries. Among Europeans, green eyes are typically more rare than blue eyes, but green eyes may be more common than blue eyes among other populations including Hispanic people, based on some studies. One study reported that 16% of Americans with Germanic or Celtic ancestry had green eyes, which makes sense because Germany and Ireland are among the countries with the highest prevalence of green eyes.
Another point to consider when examining the prevalence of eye color is the population’s generation. One study found that blue eye color among non-Hispanic white participants in the United States decreased from 57.4% in individuals born between 1899 and 1905 to 33.8% for individuals born between 1936 and 1951. It has been speculated that this decrease in the prevalence of blue eyes may have been due to increased immigration or marriages between individuals of different ancestry. Any study analyzing the current prevalence of green eyes, or any eye color for that matter, would have to specify natural eye color, as colored contacts and other means to change a person’s eye color are increasing in popularity.
In summary, eye color is determined by a combination of the type and quantity of the pigment melanin in the iris and the light scattering properties of the iris. Green eye color is characterized by a low level of melanin in the iris and can be predicted by specific mutations. Green eyes are considered rare compared to brown eyes worldwide or blue eyes in northern Europe, but the exact prevalence of green eyes varies greatly between populations (from 0 to approximately 20%). Green eyes are most prevalent in individuals of European ancestry and are typically not found in individuals of African or Asian ancestry, although there are exceptions. Countries with high prevalence of green eyes are Ireland and Germany. Green eyes are most similar and often considered the same eye color as hazel eyes, thus the prevalence of green eyes (excluding hazel eyes) is difficult to determine. Green eyes are not as rare as red or violet eyes, which are found in people with albinism, or heterochromia (eyes of different color).