The Science of Sight Ed 10: Why Do We Have Different Coloured Eyes?

The Basics of Eye Colour

Eye colour is one of the first things people notice about us, and it’s a trait that can vary widely from person to person. From deep brown to sparkling blue, or even a rare green, the colour of our eyes is a unique characteristic that contributes to our individual appearance. But why do we have different coloured eyes? What determines whether someone’s eyes are brown, blue, green, or hazel?

Eye colour is determined by the amount and type of pigment in the iris—the coloured part of the eye. The iris controls the size of the pupil and, as a result, the amount of light that enters the eye. But it’s the pigments within the iris that give our eyes their distinct colour.

Melanin and Eye Colour

The primary determinant of eye colour is melanin, a pigment also responsible for the colour of our skin and hair. Melanin comes in two forms: eumelanin (which is brown-black) and pheomelanin (which is yellow-red). The concentration and distribution of these pigments within the iris determine the colour of your eyes.

Brown Eyes: People with brown eyes have a high concentration of melanin in their irises, which absorbs more light and reflects less, giving the eyes their darker colour.

Blue Eyes: Blue eyes contain less melanin, allowing more light to be scattered by the iris. This scattering of light, known as the Tyndall effect, is what makes blue eyes appear blue.

Green Eyes: Green eyes have a moderate amount of melanin, with a combination of both eumelanin and pheomelanin, leading to the characteristic green hue.

Hazel Eyes: Hazel eyes are a mixture of brown and green, with varying amounts of melanin creating a multicoloured effect.

Genetics: The Role of Inheritance

While melanin is the primary factor in determining eye colour, genetics ultimately dictate how much melanin is present in your iris. Eye colour is a polygenic trait, meaning it’s influenced by multiple genes rather than a single gene.

The most well-known genes associated with eye colour are OCA2 and HERC2, both located on chromosome 15. The OCA2 gene plays a significant role in the production of melanin, while the HERC2 gene regulates the activity of OCA2. Variations in these genes can lead to the wide range of eye colours seen in the human population.

Brown eyes are typically dominant, meaning that if you inherit the brown eye gene from one parent and the blue eye gene from the other, you’re more likely to have brown eyes.

Blue eyes are generally recessive, so two blue-eyed parents are likely to have blue-eyed children, though variations can still occur due to the polygenic nature of eye colour.

However, it’s not just these two genes that determine eye colour. Other genes can influence the final outcome, which is why siblings can have different coloured eyes despite having the same parents.

The Rarity of Different Coloured Eyes

While brown is the most common eye colour globally, other colours are less common and can be quite rare. For example, green eyes are relatively rare, with only about 2% of the world’s population having this eye colour. Blue eyes are more common in certain parts of the world, such as Northern Europe, but are still less prevalent globally.

Heterochromia: A Fascinating Condition

In some rare cases, a person can have two different coloured eyes, a condition known as heterochromia. Heterochromia can be complete, where each eye is a different colour, or partial, where there’s a section of a different colour within one eye. This condition can be present from birth due to genetic factors or can develop later in life due to injury, inflammation, or certain medical conditions.

While heterochromia is rare, it’s generally harmless and simply adds to the uniqueness of an individual’s appearance. If you notice a change in your eye colour, it’s always wise to consult an eye care professional, as it could indicate an underlying health issue.

Factors That Can Affect Eye Colour

While genetics are the primary determinant of eye colour, other factors can influence how our eye colour appears.

Lighting and Environment

The colour of your eyes can appear different depending on the lighting conditions. For example, bright sunlight can make lighter eyes appear even lighter, while dim lighting might make them appear darker. Similarly, the colours of your clothing or surroundings can affect how your eye colour is perceived.

Age and Eye Colour Changes

It’s not uncommon for eye colour to change slightly as you age. Babies are often born with lighter eyes that can darken as melanin production increases during the first few years of life. In adults, subtle changes in eye colour can occur due to hormonal changes, certain medications, or the natural ageing process. However, significant or sudden changes in eye colour should be evaluated by an eye care professional to rule out any potential issues.

Why Understanding Eye Colour Matters

While eye colour is often considered a purely cosmetic trait, understanding the science behind different coloured eyes can offer insights into your overall eye health. For example, individuals with lighter eyes, such as blue or green, may be more sensitive to bright light and at a higher risk for certain eye conditions, such as macular degeneration. On the other hand, people with darker eyes tend to have more natural protection against the sun’s harmful rays.

At OCL Vision, we’re committed to helping you maintain healthy vision, no matter your eye colour. Regular eye exams are essential for detecting any potential issues early and ensuring your eyes remain healthy throughout your life.

Trust OCL Vision for Comprehensive Eye Care

Different coloured eyes are just one of the many fascinating aspects of human biology. Whether you have deep brown eyes, striking blue eyes, or rare green eyes, each colour is a result of a complex interplay of genetics and biology. At OCL Vision, we celebrate this diversity and are dedicated to providing top-quality eye care tailored to your unique needs.

If you’re curious about your eye colour, or if you have any concerns about your vision, contact OCL Vision today to schedule a consultation. Our expert team is here to help you understand and care for your eyes, no matter what colour they are.