Have you ever wondered why are all babies born with pretty much the same eye color – dark blue-grey? And, if you saw a recent Alice in Wonderland movie, why does one set of sisters, Red and White Queen, has identical brown eyes, but the other set of sisters doesn’t? Alice has hazel eyes and her sister has grey eyes.
In humans, eye color is determined by the amount of pigment, specifically melanin, in the iris and the density of other iris cells that either mask or expose cells containing melanin. When babies are born, melanin has not yet been developed by the iris melanocytes – cells that produce melanin. Over time, and with exposure to sun, melanin production increases. Eyes that turn out brown, have many melanocytes and produce a lot of melanin. Eyes that turn out blue, produce much less melanin. Light grey eyes make even less melanin than blue eyes. The eyes that have no melanocytes look pink (remember the albino rabbit in Alice in Wonderland?) because the blood vessels in the back of the eyes are visible through the transparent iris and they reflect light.
Eye color stabilizes by about one year of age, but can change up until three years. Changes can also happen during puberty, pregnancy, trauma, and when taking certain medications, like the one used in the currently popular eye-lash growth enhancer, Latisse. The active ingredient of Latisse, bimatoprost , when used as an eye drop to treat glaucoma or high pressure in the eye, has been associated with darkening of eye color due to increase in melanin production by the iris.
Inheritance of eye color
Eye color is determined by the interaction of multiple genes, at least six. Gene combinations that result in different eye colors are known. So, it is possible to map the genome of the unborn fetus and predict their eye color with 90% accuracy.
Even though there are at least six different genes involved, a recent scientific article estimated that 74% of most common human eye colors, brown and blue, can be explained by only one gene, called OCA2 gene. In fact, people with blue eyes have one common ancestor. Originally, we all had brown eyes. Then, a mutation arisen in a single individual in the Near East or around Black Sea region 6,000-10,000 years ago. This mutation was in the gene that either turns on or turns off the production of melanin by the OCA2 gene. When the mutation is severe, melanin production is turned off completely and albinism results. When the mutation is less severe, melanin production is reduced and blue eye color results. Most of the time two brown eyed parents will have a brown eyed baby and blue eyed parents will have a blue eyed baby.
The more rare eye colors – blue eyes with brown spot, grey eyes, and green eyes – probably involve more than one gene. Therefore, if one parent is green eyed and the other one has blue eyes with brown spots, it’s almost impossible to predict the eye color of their children without resorting to genetic mapping of the fetal chromosomes.
Eye colors in different regions around the world
(Guess the celebrity behind these eyes)
- Brown eyes – over half of the world’s populations has brown eyes. Dark pigment brown eyes are common in East Asia, Southeast Asia, Oceania, Africa, South America and the Middle East. Light-pigmented brown eyes, sometimes referred as “honey eyes,” are common in Europe, Northern India, and Middle East. Large amount of melanin in brown eyes absorbs shorter and longer wavelengths of light.
- Blue eyes – are most common in Northern Europe and Central Europe and to a lesser degree in Southern Europe and southern Central Asia; Afghanistan is a notable example. They are also found in parts of North Africa, West Asia, and South Asia, in particular the northern areas of India and Pakistan. It can rarely occur as far south as Sri Lanka.
- Gray eyes – are most common in Russia, Sweden, Finland and the Baltic States. It can also be found to a lesser extent in parts of India. Grey eyes have the least amount of melanin and are most likely to “change color” depending on the lighting, make up, and clothing.
- Green eyes – are most common in Northern and Central Europe. They can also be found in parts of South Asia, West Asia, and North Africa. In Iceland, 89% of women and 87% of men have either blue or green eye color. Among European Americans, green eyes are most common among those of Celtic and Germanic ancestry.
- Hazel eyes – are commonly found in Europe, the Middle East, the Americas, Central Asia, and parts of South Asia. Hazel eye color has the most variation of shades, depending on the amount of brown vs. green color present. Sometimes, the green is revealed only when the light shines on the eyes in a certain way, just like it does in only a few scenes on Alice’s eyes in Alice in Wonderland.
Who do these eyes belong to?
(Lucy Liu, Reese Witherspoon, Angelina Jolie, Julienne Moore, Rihanna)
Pacific Vision Institute www.pacificvision.org