What do you know about eye colours?

Have you ever envied some celebrities like Emma Stone who own a pair of attractive green eyes? Have you ever wondered why some animals have unusual attractive eye colours that we humans don’t have naturally? Before we explore, why not do a simple survey to test how much you already know?

green eyes

Poll

What eye colour don’t humans have naturally?
Brown
Blue
Orange
Green
Hazel
Golden
Grey
Violet

Quotes To Know

 

Vote: https://goo.gl/kQFkL3
Results:https://goo.gl/VlGAaD

Do you want to own a pair of wacky-coloured  bright yellow or gold eyes for a day or two? Fret not, thanks to the cutting-edge in Science and technology, you can purchase any coloured contact lenses according to your desires, with ease.Animals such as cats and owls have yellow or gold eyes that is not innate in humans. The iris within the eye is responsible for the eye colour. The iris contains melanin, which is a dark brown to black pigment occurring in the hair, skin, and iris of the eye in people and animals.If the iris contains a lot of melanin, then the eye will appear brown. On the contrary, if the melanin decreases, eye colour shows up as hazel, green or blue.

eye-colour-gif

melanin
Structure of the eye

Factors affecting eye colourPicture1.png

Amount and type of melanin in the iris

There are several types of melanin present in the eye, namely,  pheomelanin which represents more red and yellow colours and eumelanin which represents brown and black colours.There is no green or blue pigment ever present in the eye. Hence, there is only one kind of pigment, melanin, and its derivatives. So how can a pigment that only produces shades of brown or red create eyes that look green or blue? While the amount and type of melanin play a crucial role,the amount of light that penetrates also has a part to play.

eumelanin-and-pheomelanin

Lighting conditions

There are 2 main layers in the iris, a front(endothelium) and a back layer(epithelium) and in between these layers is a thin layer of tissue called the stroma, which is rich in proteins.Everyone has some kind of pigment in their iris, which usually includes a layer of melanin on the back of the iris. So, actually every individual has the same eye colour. However, the difference comes with how it’s perceived, which is due to the amount and type of melanin in the front layer of the iris and how light interacts with it.

an-cornealayers
Layers in iris

Genes

Genetics determines how much pigment is present in the iris of one’s eye. Although there are up to 16 different genes that influence the eye colour, though there are two main genes that have the most influence. They are known as the OCA2 and HERC2 genes.The amount and quality of melanin produced in the iris are mostly affected by these 2 genes.

Density and composition of stroma in iris

Different composition of stroma causes light to scatter differently, affecting the colour of the eye.

Discussion of eye colours

Blue eyes

People with blue eyes have little or no melanin on the front layer of the eye, so as light goes through the eye, it hits the back of the iris and then reflects out. As it enters the stroma, the presence of proteins causes blue light to scatter, which makes the eye look blue.

Grey eyes

Gray-eyed people have even lesser amounts of melanin in their eyes than blue-eyed people. Additionally, the different composition of stroma that causes light to scatter differently contributes to grey eyes.

Brown eyes

Those with brown eyes have melanin in both parts of their irises, so the effect of the light-scattering is not so obvious. The eyes appear darker when more light is absorbed, and variations in  shades of brown depend on the amount of melanin present.

Green eyes

Green-eyed people have only tiny amounts of melanin in the front iris layer, which tends to be the red or yellowish pheomelanin. Since the melanin concentration is pretty low, the light scattering effect gives off a blue colour, which mixes with the yellowish color of the pheomelanin, making the eye look green. Wow!

Amber eyes

Those with amber eyes get their colour predominantly from the presence of pheomelanin in the iris.

Red or violet eyes

A little misconception is that albinos are often though to have violet or red eyes. However,albinism is a condition that causes people to have a lack of pigment in their hair, skin and eyes. Since these people  lack pigment in their iris, light bounces off the back of the eye and exit the eye.

In this case,light usually reflects back red because of the blood vessels at the back of the retina. Eyes can look violet when this red color combines with the bluish color of the iris that results from a lack of melanin, and from light-scattering effects.

This is the same reason why the eyes may look red in a photograph at times, which is due to light reflecting off the back of the eye and passing back out through the iris. In normal eyes and lighting conditions, light cannot exit the eye like this.

Eye colours from most to the least rare and some characteristics

Rareness of eye colour Melanin presence on front layer of iris Melanin presence on back layer of iris Type pf pigment that is dominant
1. Green More than blue eyes, less than brown Normal Pheomelanin
2. Violet/red None or very little None or very little NA
3. Amber Heavy Normal Pheomelanin
4. Blue Light Normal Eumelanin
5. Grey Lesser than blue Normal Eumelanin
6. Hazel More than green, less than brown Normal Pheomelanin or Eumelanin
7. Brown Heavy Normal Eumelanin

In a nutshell

Although the most rare eye colour is green and the most common eye colour is brown, there are some eye colours such as bright yellow, orange or gold which humans don’t possess. Regardless of whether an iris is dilated or constricted to accommodate light, it’s highly impossible for humans to have the same eye colors as owls or cats.To see such extreme hues would require significant outliers with significantly abnormal colors for humans to find each other and mate. This of course is quite far-fetched.

owl.jpg

So if you had chosen either orange or golden as your answer in the poll, BINGO! You got it!

References:

  1. https://ghr.nlm.nih.gov/primer/traits/eyecolor
  2. https://www.poll-maker.com/#QP1013111x352424c8-42
  3. https://owlcation.com/stem/Rarest-Eye-Color-in-Humans
  4. http://www.livescience.com/56000-why-people-dont-have-orange-golden-eyes.html
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