Green Eye colour explained
St. Paddy's Day comes and goes, but Irish green eyes are around us every day. Most of the time, we're so busy going on about our lives that we're not observing and looking at people closely enough to observe those beautiful green eyes.
Genetics Are No Guarantee on Eye Colour
At first, it was believed that eye colour was simply decided by one gene or another. Now, it's clear that polygenic traits from multiple genes determine the colour of a person's eyes. That's because those traits influence the amount of melanin in our eyes. They also influence things like our height, skin colour and hair colour. The part of the eye that gives off colour is the iris. It's the part of the eye that surrounds the pupil. The iris has cells with brown coloured pigments that ultimately determines our eye colour.
Eye Colour at Birth is No Guarantee of Eye Colour
Most people who have green eyes were not born with them. It's more likely that in their infancy, their eye colour was either grey or blue. After several months or even years, their eye colour then transitioned from grey or blue to green. After that, barring any disease, they will stay green.
Melanin and Eye Colour
Pigments impart black, white or a colour to other materials or substances. When light is transmitted or reflected, pigments absorb the wavelength of that light. How much of that light is absorbed by pigments determines colour. Melanin is a term that is used for that group of pigments that are found in the body and in the iris of the eye. It's made from a certain amino acid that is known as tyrosine. The iris is the part of the eye that circles the pupil and gives the eye its colour.
The way that light scatters across your eye pigment determines eye colour. People with very little iris pigment will have blue eyes. Although there are a variety of eye colours, there are few eye pigments, and almost all eye colours result from how much melanin there is in the iris. Somebody with a large amount of melanin in their iris will have brown eyes, and somebody with just a little bit will have blue eyes. Those who have a little bit of melanin coupled with a yellow pigment called lipochrome will have blue eyes. The melanin content of the iris ordinarily remains about the same during the course of a person's life.
Green Eyes and the Silk Road
The most prevalent and current theory of what determines eye colour seems to suggest that there could be up to 16 genes that influence what colour a person's eyes will be. Although that could mean that a baby might be born with eyes of any type of colour, heredity still plays a role in eye colour. We already know that people with green eyes lived in and around Eurasia more than 2,000 years ago. Europe and Asia were connected by mountains between the Black Sea and the Caspian Sea, and because the silk traders from Asia used that route, it became known as the Silk Route. As a result of it developing into such an important route, children born of Eurasian heritage are likely to have been conceived and born from this genetic combination. That probably explains why many sets of green eyes are found in Asia, Africa and Europe.
Now, any race, anywhere around the world can have people with green eyes. In Liqian, a village in Western China, up to two-thirds of the population have green eyes and blonde hair. DNA testing showed that about 66 percent of the people in the village had Caucasian heritage. This fact might be attributable to Roman soldiers who moved through or settled into the area sometime around 100 B.C. The largest population of people who have green eyes is in Ireland, Scotland and Northern Europe. As per Ireland and Scotland, about 86 percent of the total population have green or blue eyes.
What is Genetic Anthropology?
Anthropology involves the scientific study of people, their behaviours, societies and migration patterns over the course of history. Genetic anthropology involves DNA testing, and it helps to reveal thousands of years of migration patterns. Through genetic anthropology, you might even learn why your eyes are a certain colour. Since people can now have DNA testing performed for the purpose of learning their genetic background or polygenic traits, they might also learn something about where their eye colour comes from. Any eye colour other than brown is likely the effect of genetic migration.
Green Eyes and the Sun
People with green eyes are more likely to be affected by the sun's ultraviolet rays (UVS). This goes back to the amount of melanin content in the iris. Given that lack of pigment, people with green eyes are cautioned to protect them from the sun's UV rays more than people with brown eyes. To help protect the eyes from the damaging effects of the sun you should wear maximum UV-protected sunglasses. As per the American Academy of Ophthalmology, that requires a UV protection rating of 400 or higher.
How Green Eyes Can Change
We know that eye colour is influenced by how light scatters on the eye's iris. Here are a few other factors that might play a role in the shade of green that a person's eyes are:
● Artificial light from bulbs. How intense that light is can intensify or diminish the green in a person's iris.
● What time is it? Intense sunlight at noon is more likely to affect an iris than sunlight later in the day.
● Colours on you or surrounding you. Blues can make eyes look bluer and greens can make eyes look greener.
● Medications and street drugs. These are known to directly affect the size of a person's pupils. The use of medications or street drugs might result in eye colour appearing more intense.
● Emotions: A person's eye colour isn't indicative of his or her emotional state, but watery eyes or even crying eyes are likely to cause for pupils to dilate, as a result, the colour of a person's eyes might appear to be deeper.
On rare occasions, a condition known as heterochromia is seen. For example, a person might have one blue eye and one green eye. Genetic mutation or injury are possible causes. David Bowie acquired heterochromia after being hit in the eye at an early age. Actors Judd Hirsh and Kiefer Sutherland were born with the condition as were actresses Kate Bosworth and Jane Seymour
How Green Eyes Might Change Colour
Glaucoma involves increased pressure on the front of a person's eye from the accumulation of fluid. As a result of that excess fluid, pressure is put on the optic nerve. Most glaucoma cases develop over a period of years, but some develop rapidly. It's one of the leading causes of blindness. Glaucoma sufferers must be cautious about any medication that they take for the condition. Some medications can change the colour of a person's eyes. Other conditions like Horner's syndrome or Fuchs dystrophy in and of themselves might cause eyes to change colour.
The Truth About Green Eyes
Assuming that eyes are the window to the soul, then you would expect the colour of a person's eyes must speak of their personality. Nothing could be farther from the truth.
Green eyes can be associated with jealousy, extroverts and even evil, but these falshoods often have their roots in ancient history and Greek mythology. If you have green eyes, congratulations, you are pretty unique, with only approximately 2% of the world's population sharing your eye colour.
Given the remarkable advancements in genetics and DNA testing over the past 25 years, we continue to learn more about eye colour and green eyes. For now, if you're not satisfied with the colour of your eyes, you can always see an eye care professional and try contact lenses of a different colour. If you have perfect eyesight, you can wear plano coloured lenses that will change your eye colour but not affect your vision.
Author: John Dreyer Optometrist Bsc(Hons), MCOPTOM, DipCLP
Created: 26 Apr 2023, Last modified: 27 Apr 2023