Practice News

Why Do People Have Different Coloured Eyes?

Why do some people have brown eyes, or blue, maybe grey or even green? What decides the colour and how? Some people even appear to have a mixture which appears to change according to the light and very occasionally, you might even meet someone whose eyes are both different colours.

It’s all in the iris

The part of our eye where we see these colours is the iris. The iris sits behind the cornea of the eye and is responsible for controlling the amount of light entering the retina, which it manages by changing the diameter and size of the pupil in reaction to the amount of light it is exposed to (which is why your pupils contract in bright light and expand in low light). The iris itself is made up of several layers, with the outermost two collectively referred to as the anterior border. These are the layers that contain the cells called melanocytes and their job is to produce a pigment known as melanin.

Much like our skin tones and natural hair colours, the colour of the iris in our eyes is dependent on the amount of melanin present. How much melanin an individual produces is determined by a combination of several genes which we inherit from our parents. The more melanin we produce, the darker and browner our eyes will generally be, in the same way that more melanin generally means a darker skin tone.

African origins

Scientists believe that as early humans originated from Africa, in consistently sunny climates, they all had dark brown coloured eyes and darker skin tones to protect them from the sun. It is thought that as humans migrated away from the equator into cooler, more seasonal lands that anybody carrying genetic mutations such as paler skin and paler eyes then had a better chance of survival and passing on the genetic traits to their offspring, which led to several colours, brown, blue, green, as well as more subtle ‘blends’ of these colours, resulting in gray and hazel. It is also common for people to have brown patches or rings within irises that are largely other colours.

Less melanin makes the two top layers of the iris more translucent, allowing more light through into the stromal cells below. These cells reflect the light back as blue or gray. This blue/gray light reflecting through a semi-brown layer causes mixed colours such as green or hazel. So the only colour pigment that’s ever present in any person’s eyes is brown, any other colour or colour combination is dependent on the amount of brown present and the reflection of light.

Blue eyed babies

The reason why many caucasian babies are born with blue eyes that change to another colour as they develop is because they are born with little melanin present in their eyes but once they are born, light stimulates the production of melanin. This change tends to occur most often around within the first 3 years of life, but some people’s eyes can change colour even as adults. Babies born with brown eyes will always remain brown.

A matter of luck

Some people have two eyes that appear very different colours. The medical term for this is Heterochromia Iridium. It is rare, affecting about 1% of the general population. It can be caused by external factors such as trauma to the eye, certain medical conditions and medications or it can be simply a result of genetic ‘pot luck’. While scientists have continued researching the genetics around eye colour, much is still unknown as it involves complex interactions of at least 16 different genes. Just because both parents may have the same eye colour does not guarantee any child will inherit the same. So, for all the new parents out there, there is no accurate way of predicting a baby’s eye colour as they grow other than to wait and see.  

The team at Sergeant & Barber Opticians would like to thank all of our patients for choosing us in 2018 and we look forward to seeing you again in 2019. We wish you the very best for a merry Christmas and a happy new year.

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