Why Do We Have Different Coloured Eyes?

close up of one eye

Have you ever wondered what determines the colour of our eyes? Or why we don’t always have the same eye colours as our parents or siblings? There are six main eye colours:

  • Brown
  • Amber
  • Hazel
  • Green
  • Blue
  • Grey

Despite fitting into one of these categories, our eye colour is made up of dozens of genes and this creates subtle differences from one person’s eyes to the next. This means that your eye colour is as unique as your fingerprints!
We’re going to break down the reasons for our different eye colours, what determines our eye colours, and conditions that can affect eye colour. We’ll also debunk a myth or two.

Why do we have different eye colours?

It’s all down to pigmentation. The colour of our eyes is determined by the amount of melanin we produce, the same melanin that determines the colour of our skin. The coloured part of the eye is called the iris and it’s located behind the cornea. The iris is made up of several layers and helps control how much light enters the eye.

The two outer layers are known as the anterior border and this area contains pigment-producing cells called melanocytes. We all have roughly the same amount of these melanocytes, but how much melanin they produce differs from person to person. The more melanin we produce, the darker the colour.

The most common eye colour worldwide is brown. So, if you have brown eyes, it’s because you have a lot of melanin in both layers of your iris. If you have hazel or green eyes, it means you have less melanin in the front layer. And if you have blue or grey eyes, you have little to no melanin in the front layer. Light passes through your anterior border and interacts with grey cells further into the iris and the light reflected back is what gives your eyes a blue colour.

It’s believed that it was a single genetic mutation that led to the variation of the greys, blues and greens – a change in the gene that produces melanin – and that it can be traced back to a single, common ancestor. It is also speculated that the subsequent spread of this eye colour has evolutionary significance. For example, darker eye colours with higher levels of melanin are often found in hotter, sunnier climates. While lighter eye colours, with lower levels of melanin, are more common in countries with colder climates. This theory is known as the Vitamin D hypothesis and it suggests that light-coloured skin, hair and eyes evolved as humans moved into latitudes with less sunlight.

Are all babies born with blue eyes?

No. While it is common for babies to be born with blue eyes, it’s not guaranteed. A 2016 study from Stanford University found that only 20% of babies are born blue-eyed. Around 63% actually have brown eyes, and just under 6% of them have green eyes. Many of those blue-eyed babies actually lose the blue within a few months.

Why do I have different coloured eyes to my sibling?

Eye colour is a polygenic trait. This means it is determined by multiple genes and the interactions between them. Up to 16 genes can influence eye colour, and siblings can inherit various genes from their parents with no guarantee that they’ll each get the same ones. An individual’s eye colour will depend on which genes they have inherited which is how one sibling might have light, blue-coloured eyes and the other could have dark, brown-coloured eyes.
In general, the gene that causes brown-coloured eyes is dominant over the blue-eyed gene which is why brown eyes are more common around the world. This often means that if one parent has blue eyes and the other has brown eyes, there’s a good chance that their child will have brown eyes.

Health conditions that affect eye colour

Albinism is a condition that not only affects the colour of skin and hair, but it can also affect eye colour. Variations of albinism result from mutations in the genes involved in producing and storing melanin. People with albinism have little or no melanin in their eyes, often resulting in very pale blue or grey eyes. If there is no melanin in either layer of their irises, the blood vessels in their retinas become much more visible, which is what results in a shade of pink or red. The reduced pigmentation of the iris caused by ocular albinism leads to very light-coloured eyes and often problems with vision, which can be helped by prescription glasses.

Eye colour can often be linked to a person’s risk for certain diseases. People with brown eyes are more at risk of developing cataracts, and people with blue eyes have a higher risk of developing macular degeneration, cancer of the eye, or diabetic retinopathy. This is possibly caused by less internal protection in the eye. But it’s not all doom and gloom for blue-eyed people because they’re said to be more resistant to the effects of seasonal affective disorder!

Can a person have two different eye colours?

Yes. This is called heterochromia and it is when an individual has two different-coloured eyes. The condition can be categorised into three main types:

  • Complete heterochromia – where each eye has an entirely different colour. For example, one might be blue and the other might be brown.
  • Sectoral heterochromia – also known as partial heterochromia, this involves variations in colour within a single iris, often creating a unique pattern in the eye.
  • Central heterochromia – involves a ring pattern of different colours going from the pupil towards the outer iris.
    While some cases of heterochromia are hereditary, others can occur without a clear familial link. They could have been caused by genetic changes, by a problem during eye development, or as a result of disease or injury to the eye.

If you have noticed a change in the colour of your eye, you’d benefit from a comprehensive eye exam as it could be a sign of injury or a health condition. Don’t hesitate to contact us today to speak to a professional, or find the contact details for your local independent opticians and book an appointment online now.

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