Understanding the Origins of Eye Colour

Ever heard the phrase ‘your eyes are the windows to your soul’? It’s a common enough saying, but what many people don’t realise is that there may be some truth in it. Your eyes might not give anyone any particular insights into the depths of your soul, but they can tell us a story about your eye health, such as signs of redness and irritation, your genetics, and even your family history. 

In this article, we’ll explore the science behind eye colour, touching upon the genetics which determine eye colour and eye pigmentation. We’ll also look at eye conditions that can permanently alter the colour of your eyes. 

Genetics 

Your genes play a key role in determining the eye colour you are born with. Scientists originally believed eye colour was determined by a simple inheritance pattern, with brown eyes always being dominant over blue. However, recent research has shown that this form of thinking is too simplistic, with multiple genes involved in creating eye colour and that this feature can even be influenced by the genes of more distant relatives rather than just your parents.

There are two types of genes responsible for eye colour. The first is the OCA2 gene which directly determines how much brown pigment (melanin) is present in the iris. OCA2 gene is influenced by the 2nd gene known as HERC2 – which acts as an on/off switch – together they impact how much pigment is produced in the iris, resulting in brown, blue or other light-coloured eyes.

It is believed that at one time every single human had brown eyes. However, due to a gene mutation, people started to produce less melanin, leading to other eye colours. Those with less melanin in their eyes absorb less light. Melanin protects the eyes from sun damage which might be why you commonly see more brown eyes in continents that typically have warmer weather.

It is estimated that 70-80% of the world’s population have brown eyes. Meanwhile, those with blue make up around 8-10% while green eyes only make up 2% of the population. This makes green one of the rarest eye colours.

Estimated Eye Colours Globally:

• Brown (70-80%)
• Blue (8-10%)
• Hazel (5%)
• Amber (5%)
• Grey (3%)
• Green (2%)

 

Albinism

Usually passed down from parent to child, Albinism is a rare genetic condition where the body has trouble with creating pigment, this genetic mutation is usually passed down from parent to child. People with albinism typically have either no melanin at all or reduced levels of the pigment usually have reduced or are completely absent of pigment in their eyes, hair and skin. 

There are two main types of albinism: oculocutaneous albinism (OCA) and ocular albinism.

• OCA affects the eyes, skin, and hair, causing them to have less colour.
• Ocular albinism mainly affects the eyes, while the skin and hair usually look normal.

There are many secondary symptoms of albinism beyond the visible lack of pigmentation. They include:

• Nystagmus – eyes moving rapidly and uncontrollably.
• Photophobia – light sensitivity.
• Foveal hypoplasia – a condition where the retina doesn’t develop correctly during birth and infancy.
• Iris issues – due to the lack of pigment, the eye struggles to keep out stray light.
• Optic nerve issues – the signals sent from the retina to the brain do not follow their usual nerve path.

Albinism is typically caused when both parents carry the mutated gene. However, as the gene is recessive, it is unlikely a child will develop the disorder if only one of the parents carry it.

Blue Eye Theories

We know brown eyes make up the largest proportion of eye colour globally, and that the brown pigment comes from melanin. But why do many countries in Europe, including the UK, have blue as their most dominant eye colour? 

Blue eyes have been around for at least 7,000 years, and there are a few different scientific theories as to why this might be.

Vitamin D Theory:  Lighter skin, hair, and eyes developed as humans migrated away from the equator to areas with less sunlight, therefore needing less melanin. This theory has limits due to the fact that blue eyes are not proven to be any more efficient at seeing in low light conditions.

Seasonal Depression Theory: Blue-eyed people may have some protection against Seasonal Affective Disorder (SAD), a type of depression linked to lack of sunlight in winter. Lighter eyes are more sensitive to light, which might help melatonin and serotonin regulation in the body during darker months. 

Attraction Theory: Some researchers suggest that rare eye colours, like blue, are considered more attractive, possibly due to their uniqueness. As eye colour is hereditary, sexual selection could increase the frequency of blue eyes over generations.

Changes in Eye Colour

It is possible for an individual’s eye colour to change throughout their life. This can sometimes be down to a person’s eye health deteriorating due to disease or injury.

Cataracts

Cataracts happen when the lens inside your eye becomes cloudy. This is more common as we get older. Normally, the lens is clear, but over time, it can become hazy. While cataracts are more common in adults, they can also affect babies and children. Usually, both eyes develop cataracts, but not at the same time.

What Causes Cataracts?

Several factors can increase your risk of developing cataracts:

• Family history of cataracts
• Medical conditions like diabetes
• Certain medications, such as corticosteroids
• Eye injuries, surgeries, or radiation treatments
• Smoking
• Symptoms of Cataracts

If you experience any of these symptoms, it’s important to see an eye doctor:

• Blurry vision
• Double vision
• Sensitivity to light, especially at night
• Difficulty seeing in the dark
• Faded or yellow colours

Heterochromia

Heterochromia is a rare condition where a person’s eyes are two different colours. Less than 1% of the world’s population has this unique eye colour.

There are three main types of heterochromia:

• Complete Heterochromia: One eye is a completely different colour than the other.
• Partial Heterochromia: Part of one or both irises is a different colour.
• Central Heterochromia: The inner ring of the iris is a different colour from the outer ring.

What Causes Heterochromia?

Heterochromia can be caused by:

• Genetics: Most cases of heterochromia are harmless and caused by a genetic mutation.
• Medical Conditions: Some diseases, like Hirschsprung’s disease and neurofibromatosis 1, can cause heterochromia.
• Injury or Surgery: Damage to the eye can sometimes lead to colour changes.

Understanding the science behind your eye colour is not only interesting, but it also sheds light onto the importance of eye health. Always reach out to a medical professional or your local opticians if you start to notice any changes to your eyesight or eye colour.