How do you inherit green eyes?
Green eyes are especially rare. In humans, the pigmentation of an iris can range from very light brown to a very dark brown, bordering on black. The darkness of the iris depends on its concentration of melanin—the greater the concentration of melanin, the darker. Green eyes posses low to moderate amounts of melanin. However, the green is actually not a pigmentation of the iris at all. What is known as Rayleigh scattering—the same scattering of light that occurs in the sky and makes it blue—occurs in the outer layer (stroma) of the iris. The blue optical effect of the Rayleigh scattering combined with the light melanin pigmentation combines to make the eye appear green.
Only 2% of the world's population have green eyes or 140 million people. Why? The answer boils down to genetics.
Until 2007, it was believed that there was a hierarchy of eye color. Brown was dominant over green, which was dominant over blue. You received two genes for an eye color, one gene from each parent, and whichever was the dominant gene would express itself. Therefore two brown genes would obviously express itself as brown. A brown gene and a blue or green gene would express itself as brown. A green gene and a blue gene would express itself as blue. And only a combination of two blue genes would express itself as green.
The study disproving this Mendelian theory for eye color was published in theAmerican Journal of Human Genetics, and was supported with grants from Australia's National Health and Medical Research Council and the United States of America's National Cancer Institute.
Now it is proven that eye color is controlled by more than one gene; in fact, up to 16 genes contribute to eye color. This means that there is a chance that a newborn child can inherit virtually any eye color, no matter the eye color of the parents (although the chances for people of certain heritages are different than for others).
Northern Africa and countries such as Iran, Pakistan, Afghanistan, Central Asia, Spain, and Brazil all have had people with green eyes for a long time. All races, including white, black, Asian, and indigenous peoples of the Americas can also have green eyes. Green eyes most likely developed because of the migration of people over time, those from northern latitudes with light coloring traveling south and people of dark coloring traveling north intermixed leading to a variation of melanin produced.
Notably, there have been records of people with green eyes originating around the Caucasus mountains, which links Europe and Asia. Several passes through the mountain range made it a strategic region or military and trade(silk road).
Is it possible there will be more green eyed people in the future due to globalisation? For now only time will tell.
As mentioned before, green eyes actually have no green pigmentation. Instead, they have a light brown pigmentation mixed with a blue optical effect caused by Rayleigh scattering. In other words, the same scattering of light that occurs in the sky takes place in miniature within the outer layer of the iris.
Milder Rayleigh scattering results in hazel eyes.
Because of this, changes in the light being scattered by the iris of lighter colored eyes will change the apparent color of the eyes. This results in "chameleon eyes", or eyes that change color. They fall into the amber, hazel, green, grey, and blue range. Specific factors that can cause the eyes to appear to change colors are:
Mood: while mood does not change the color of one's eyes, it can change
other factors. For example, anger can dilate blood vessels, causing bloodshot eyes that make eyes appear greener through contrast. Extreme happiness or grief can dilate pupils, causing the eyes to seem darker.
Weather: the weather can change the ambient light. There is a big difference between the spectrum of colors in the light cast by a cloudy day, a sunny day, a moonlit night, and a sunset.
Temperature: temperature can seem correlated to eye color, but this is because the temperature may vary in various lighting situations—for example, the lighting in your office when it's cold and cloudy and the windows are drawn vs the lighting in your office when it is sunny and the windows are open.
Colors you are wearing: wearing certain colors such as blue, yellow, or green will provide more of that color for your eyes to scatter, enhancing that color. Green and gold eyeshadows act similarly. On the flip side, other eyeshadows that make green eyes stand out more are colors in the purple family (plum, lavender, lilac), coppers, and apricot because they contrast with the green, making it stand out even more.
Colors around you: the colors in close proximity to you will act similarly to the colors you are wearing.
Lighting: artificial lighting can vary to an even greater degree than natural lighting, and the varying spectrums of light will impact your apparent eye color
Again, drastic eye color changes, especially in adults, could be a sign of an eye problem. If they occur, please see your eye doctor.
Without Rayleigh scattering, green eyes would appear brown.
Medical Reasons for Eyes to Change Color
Source: Xavier Nájera, CC 3.0, via Wikimedia Commons
Unwanted Color Change
A color change in the iris can be caused by factors such as radiation, disease, and chemicals. For example, glaucoma medications report a side effect of permanently changing lighter colored eyes to brown. Glaucoma medication was discovered to also have the side effect of lengthening eyelashes and is marketed as an eyelash lengthener. These products are FDA approved under the supervision of a doctor.
If, as an adult, one or both of your eyes changes colors drastically, see a doctor. It could be a symptom of diseases such as Fuch’s heterochromic iridocyclitis, Horner’s syndrome, or pigmentary glaucoma.
Other health risks for people with lighter colored eyes can come from radiation. Melanin in the iris helps protect eyes from UV sunlight. People with lighter colored eyes, including green, are more at risk for sun-related eye problems including melanoma of the uvea, a cancer. People with lighter colored eyes should wear UV-protective sunglasses while outside.
Cosmetic Eye Surgery
While illegal in the United States, elsewhere in the world iris implants are available with surgery, resulting in light grey eyes. This procedure can sometimes be reversed.
While no companies offer permanent eye color change for cosmetic reasons, researchers are studying ways to change eye color permanently without the use of implants.
Eyeshadow that makes green eyes stand out more are colors in the purple (plums, lavender, lilac) family, copper, greens, apricot and gold.
Some people believe green eyes are magical.
Because only 2% of people have green eyes, it is no wonder that various superstitions have flourished around the eye color. Here is a list of some of the superstitions surrounding green eyes.
Green eyes are said to be magical.
People with green eyes tend to have long lasting relationships.
People with green eyes are the most passionate.
Green-eyed people are jealous, as in the saying "green with envy".
Green-eyed people may have liver or other digestive problems.
Green eyes are a powerful aphrodisiac.
Here is a bit of interesting trivia, take it with a grain of salt. The International Community for Alien Research conducted a study of potential alien contactees and found that green eyed people were the largest group to report contact!
"Green/Hazel eyes make up 48% of the 1400 contactees that took part in this study and make up 70% of the O Neg. group. Blue eyes making up 36% of the blood types and 23% of the O Neg. group.
Brown eyes make up 16% of the study group and 7% of the O Neg. group."
You can read more here:
http://icar1.homestead.com/Contactee-Blood-types.html
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