Brown, blue or green

Eyes are the windows to the soul and sight is our most precious sense.

Our eyes are distinctive features and often used when we describe someone. Songs have been written about bonnie blue-eyed lasses and Shakespeare sang their praises in prose.

How do we get our unique eye colour?

Eye colours depend on how much pigment resides in the iris. The coloured area at the front of the eye is called the iris. It is around 12 millimetres in diameter and has an opening in the middle, which is called the pupil. The iris is made from connective tissue and a thin muscle that allows it to open and close in response to light.

There is only one pigment that determines eye colour and that is called melanin. Melanin is a substance in your body that produces hair, eye, and skin pigmentation. The cells in the iris that make pigment are called melanocytes, and they are also responsible for the colour of our hair and our skin.

Melanocytes can make two different types of pigment: eumelanin, which is brown-black, and pheomelanin, which is red.

Shades of blue

Eyes come in many shades, ranging from dark to light brown, and from green, hazel, and grey, to blue.

Dark eyes have the most pigment, particularly brown-black eumelanin. In contrast, light blue eyes have the least amount of pigment.

However, there is no such thing as blue pigment in our eyes. Instead, an individual’s eyes are blue because of the white collagen fibres in the connective tissue in the iris. These fibres scatter light and make the iris look blue.

Eye colours that fall in-between the extremes of dark brown and light blue have varying amounts of pigment and areas without any pigment. This leads to the unique colours that we see in the form of green, hazel, and grey.

A rainbow of hues

According to estimates from the American Academy of Ophthalmology, 70–79% of the world’s population have brown eyes, making it the most common eye colour worldwide.

In fact, geneticists believe that everyone on earth had brown eyes around 10,000 years ago.

People with brown eyes are less likely to develop eye cancer, macular degeneration, and diabetic retinopathy than those with lighter coloured eyes, but are more at risk of developing cataracts as they get older.

Blue is the second most common eye colour globally, with estimates suggesting that 8–10% of people have blue eyes. Scientists believe that it is possible to trace all blue-eyed people back to a common ancestor, who likely had a genetic mutation that reduced the amount of melanin in the iris.

Approximately 5% of the world’s population and 18% of people in the U.S. have hazel eyes, which are a mixture of green, orange, and gold. Hazel eyes are more common in North Africa, the Middle East, and Brazil, as well as in people of Spanish heritage.

Amber eyes, which have slightly more melanin than hazel eyes but not as much as brown eyes, account for about 5% of the world’s population. People of Asian, Spanish, South American, and South African descent are most likely to have amber eyes.
The rarest eye colour is green, with only an estimated 2% of the world’s population having green eyes.

Close to 3% of the world’s population have grey eyes. People with grey eyes have little or no melanin in their irises, but they have more collagen in a part of the eye called the stroma. The light scatters off the collagen in a way that makes the eyes appear grey.

When there is little or no melanin in the iris, the lack of pigment causes red or violet eyes. This is of the seen in people with albinism or ocular albinism.

When a person has more than one eye colour, it is called heterochromia, and it affects less than 1% of people. One can be born with heterochromia, but it can also be caused by an eye injury or health problems.

Are eye colours genetic?

Yes and no. A child’s eye colour is influenced by the colour of their parents’ eyes. As many as 16 genes influence eye colour, but the parents’ genes can mix and match in various ways. Each parent has two pairs of genes on each chromosome, and multiple possibilities exist for how this genetic information is expressed in terms of eye colour.

At one time, brown eye colour was considered “dominant” and blue eye colour was considered a “recessive” trait. But modern science has shown that eye colour is not that simple.

The influences from each parent on eye colour are not fully known until after the child is born and begins to mature. This interesting article provides more information about eye colour genetics.

When eyes no longer shine …

According to the World Health Organisation, about 2.2 billion people world-wide suffer from vision impairment, which is defined as no or low sight. Amongst these, are people whose sight can be restored with a cornea transplant.

In South Africa, the need is great. The only option for most patients in need of a cornea transplant, is a cornea donation through deceased tissue donation.

YOU can help.

Become informed – click here to read everything you need to know about cornea donation.
Register as an organ and tissue donor and tell you family about your decision.
Spread the word. Talk to people about cornea donation and help to raise awareness.

Your gift can help restore sight and light up a world, long after you have closed your eyes for the last time.