Tuesday, 3 April 2012

The Mystery behind the colour of your eyes

Dear readers,
This sexy new post was a request from a fellow blogger: She wanted to know more about eye colours and genetics.

Let's start with a schematic of the eye so we know the different terms. I'm mainly going to write about the iris:
From MediciNet.com

Iris Structure

So the Iris is a thin and circular structure, responsible for controlling the diameter and size of the pupil and thus for the amount of light reaching the retina. 
In response to the amount of light entering the eye, muscles attached to the iris expand or contract so the pupil appears small or dilated. The larger the pupil, the more light can enter.
From HERE
Iris colour

Iris colour is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, density within the iris stroma, and the scattering of light in the stroma. 

From HERE

Only Melanin, a yellowish-brown to dark brown pigment exists in the stroma. This pigment lies in a thin but very opaque layer across the back of the iris.

Neither blue nor green pigments are ever present in the iris!

So it all depends on the lighting conditions and all the other components beside the stroma and how lights reflects/scatters and is absorbed on them - Especially if you have lighter-colored eyes, since there is no melanin pigments to enter this equation.

Now that we know this, we can understand better why some babies, usually Caucasian ones, are born blue-eyed:
It is simply because there is no pigment in the stroma yet. The blue light is what we see due to scattering and selective absorption of light from the layer at the back of the iris. With times, when melanin deposits in small or big quantity, they can change to darker colours. Also where melanin deposits and how dispersed it is has an influence on what the final colour that we'll see. 
This deposit can happen over the first THREE years of your baby's life so nothing is set in stone after 6 months like many people think, although I don't think your baby is not gonna go from brown to blue...

Note that this melanin pigment is only slightly different from the one found in skin and hair.

Genetics

OK, so we all remember from school that the Blue gene is recessive while the Brown gene is dominant (see the quick reminder on genetics below). 

And although this is not entirely false, it is not entirely true! However, as all the contributing factors towards eye color and its variations are not fully understood, I can only summarize the latest research!!

Let's start with what we know:
From Here
The darker colours tend to dominate, so brown tends to win out over green, and green over blue but let's remember that it is only statistics!

Note as well that as the pupil size changes, the pigments compress or spread apart, changing slightly the overall colour. For some people, emotions can bring out different colours.Personally, don't ask me if my eyes are green or brown because it depends!!! 

Genetically, latest research shows that

Colour is determined by many genes:

-EYCL1 - a green/blue gene located on chromosome 19
-EYCL2 - a brown gene located on chromosome 15
-EYCL3 also called BEY2 - a brown/blue gene located on chromosome 15
- and Polymorphisms on a gene called OCA2, that explains up to 74% of variations between colours, also located on chromosome 15.
- a mutation in HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes and variations.
- SLC24A4 and TYR - also implicated in colour variations.

Quick catch up on genetics:

DNA is formed of a double helix of nucleotides. 
It is "segmented" into genes that contain the information to each and every part of our body. 
In itself, and during cell replication, DNA forms structures called chromosomes. 
As humans, we have two sets, called alleles. 
The interaction between alleles can be dominant or recessive and this is going to determine the final behaviour of our genes and therefore what we see like for example the colour of your eyes.
A polymorphism on the other hand is a natural modification of a gene, like a healthy mutation, that gives extra variation on this overall behaviour.

Different colours

Amber eyes
Not be confused with hazel eyes. The former being of a solid colour due to the deposition of a special pigment called lipochrome. This colour occurs less frequently.

Blue eyes
Most common in the Baltic sea area, Northern, Eastern and Central Europe and in Southern Europe (99% in Estonia, 89% in Danemark, 75% in Germany). They are also found in the Middle East and the Jewish population of Israel. Although worldwide, they account for 2.2% of the population!

Brown eyes
Most common in East Asia, South-East Asia, South Asia, West Asia, Oceania, Africa and Americas. And Jewish and Arab populations in the Middle East.

Gray eyes
There are most common in Northern and Eastern Europe. They can also be found in Algeria, Iran, Afghanistan and Pakistan.
Are similar to blue eyes but have larger deposits of collagen in the stroma so that the light is not scattered so much.
Comparison would be blue sky vs cloudy sky, we perceive the sky gray and not blue because the light reflecting on the dark clouds scatters differently on the water droplets.

Green eyes
Most common in Northern and Central Europe, particularly in Iceland.
Their appearance is a combination between the melanin pigmentation and the blue tone due to light scattering. They also result from multiple polymorphisms in OCA2 and other genes.

Hazel eyes
Similar to Green eyes in term of combinatorial effect. They can appear to shift from green to brown and this can sometimes produce a multicoloured iris (light brown near the pupil and darker green in the iris, or vice versa)

Red and Violet eyes
This is due to extreme low quantities of melanin, in a condition called albinism, allowing the blood vessels to show through. 
Note: Red eyes in photos are also due to this effect.

Changing your eye colour

  • The easiest is by the use of contact lenses
  • In the future, we'll be able to do this by surgery: 
From HERE

By removing the melanin pigments; making the resulting eye colour either blue or green - depending on the other components of the iris. 
However it is not FDA-approved yet and more tests need to be conducted. The company that does this is called Stroma Medical. They claim that the procedure, that would cost you $5000 is only 20 seconds long and it takes 2 to 3 weeks for the effects to appear, the results being permanent!!! Scary!! And let's remember that this idea started in Auschwitz by a certain Dr Mengele...





Here you go, that's it for the sexy colour of your eyes. If you liked it, don't hesitate to leave me a comment and share this page on FB, G+ or Twitter ;)

4 comments:

  1. Interesting! Thanks. Blue eyed Donna

    ReplyDelete
  2. Quite interesting and informative...

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  3. My husband is Japanese and has Dark Brown eyes whereas I am European, and have Ice Blue eyes. I did this type of research also while pregnant hoping that my children may have green or hazel eyes. The Japanese won out. Both of my children have Dark Brown eyes. >.<

    ReplyDelete

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