What determines eye colour?

Sharon Copeland Sharon Copeland
Monday, 19 April 2021 Share this blog: Facebook Twitter LinkedIn Copy link Copy Link

Eye colour is often one of the first things we notice about a person’s appearance. A person’s eye colour is a very unique feature, and the concentration of colour differs from person to person. This article looks at what determines eye colour.

How eye colour develops

Eye colour develops during the first six months of a baby's life when pigment epithelial cells start to pump three pigments into the stroma. Eye colour is usually fully developed by the time a baby is one year old.

The coloured part of the eye is known as the iris and contains a combination of pigments in the layer known as the stroma. The combination of pigments, the way in which they are dispersed and how the stroma absorbs them determines the colour of the eye. There are three pigments in the stroma:

1. Melanin: This pigment also determines skin colour and is yellow/brown.

2. Pheomelanin: This pigment is red/orange and is responsible for red hair, and is predominant in green and hazel eyes.

3. Eumelanin: A black/brown pigment that exists in large quantities in dark eyes. This pigment determines hue saturation.

It’s not uncommon for many babies to be born with blue eyes and for them to darken as they grow a bit older. Eye colour darkens if melanin develops.

Newborn babies' eyes usually correlate to their skin tone. Caucasian babies are often born with blue or green eyes, while black, Hispanic and Asian babies typically have brown or black eyes.

While children can be born with completely different eye colours to their parents, a child will likely have brown eyes if both parents have brown eyes as this is a dominant eye colour.

Some children are born with heterochromia, a condition where the irises differ in colour. This can be caused by a range of factors, including trauma in the womb or shortly after birth, inflammation and Horner’s syndrome.

Is eye colour determined by genetics?

Genetics and the chromosomes inherited by a child carry genetic information which determines eye colour. Each cell in the human body typically contains 23 chromosomes. Chromosome 15 features 600-700 genes which are necessary to produce proteins. Two of these genes involved in producing proteins are known as OCA2 and HERC2. They play major roles in eye colour.

OCA2 produces the protein responsible for more melanin production. However, the HERC2 gene can turn the protein production of the OCA2 gene on and off. If there is a genetic variation in the HERC2 gene, the amount of melanin produced can be reduced, which can lead to lighter eyes,

Brown eyes have more melanin than green or hazel eyes. Blue eyes have very little pigment, and no melanin results in very pale blue eyes. When someone has no melanin, this is known as oculocutaneous albinism.

While eye colour is determined by genetic makeup and is passed down through generations, genetic variations can sometimes cause surprising eye colours.

Can a baby’s eye colour be predicted?

baby eyes

It is possible to make fairly accurate predictions about what colour a baby’s eyes will be. While the likelihood of certain eye colours increases based on your parent's eye colour, predictions are never entirely certain. For example, if a baby has blue-eyed parents, it's more than likely that they will have blue eyes.

The below table works as a baby eye colour calculator. Simply follow both parents' eye colour, and where both join, you'll be able to see what colour a baby’s eyes are likely to be.

Heterogenous inheritance (more likely)


BrownBlueGreenHazel
Brown75% brown50/5050/5050/50
Blue50/5099% blue50/5050/50
Green50/5050/5099% green50/50
Hazel50/5050/5050/5099% hazel

Homogenous inheritance (less likely)


BrownBlueGreenHazel
Brown99% brown99% brown99% brown99% brown
Blue99% brown99% blue50/5050/50
Green99% brown50/5099% green50/50
Hazel99% brown50/5050/5099% hazel

Can eye colour change?

Once eye colour is set, it’s very unlikely that it will change. Nevertheless, there are many factors that can influence eye colour and whether or not it changes due to a different pigment.

Increased sun exposure

Exposure to sunlight causes more melanin production in our bodies, and this can cause our eye colour to change slightly if our eyes are exposed to more sunlight. As a result, they may appear a darker shade than our current eye colour.

Natural sunlight can also reveal colours that already exist in your eyes as it is brighter than artificial light.

Changes in Pupil Size

The size of your pupils can also cause changes in eye colour. For example, when our pupils dilate, our iris' are less exposed, causing them to look darker.

When our pupil's shrink, the colour of our eyes may appear lighter due to the contrast of the black of the pupil and the iris surrounding it.

Many people believe David Bowie to have two different coloured eyes due to heterochromia, but he actually has anisocoria, a condition which caused his left pupil to appear larger and therefore darker than his right pupil.

Clothing and makeup choices

Certain colours of clothing and makeup can enhance our eye colour. Your eyes can appear lighter or darker depending on the colour you wear.

Your eye colour can also change if you wear coloured contact lenses, whether they’re prescription or cosmetic use.

Age

Eye colour can change with age, especially in those with lighter eye colours. For example, it’s not uncommon for brown eyes to become hazel or for some hazel eyes to even darken with age.

However, if you experience dramatic changes in eye colour, you should see your doctor immediately as this could be a sign of certain diseases such as Horner's syndrome or glaucoma.

Emotions

Certain emotions can make your pupils change size and your iris’ change colour. When you’re angry, sad, or happy, your body releases a hormone that can change your pupils' size. When we cry, our eyes become a reddish colour making them appear brighter in colour. When we are happy or angry, our eyes usually appear more vibrant.

Can you have two different coloured eyes?

It is possible to have two different coloured eyes. There are many celebrities with different coloured eyes. This condition is known as heterochromia and is caused by variations in the concentration and distribution of melanin.

heterochromia

There are different types of heterochromia. Central heterochromia can cause different colours within the same eye, while complete heterochromia causes two completely different coloured eyes.

Heterochromia can also be a result of an injury, illness or medication and develop later in life. This is known as acquired heterochromia.

What does your eye colour mean?

There is meaning behind the colour of human eyes, and the below explains why there are brown, blue, green and hazel eyes.

Brown eyes

Brown eyes are the most common eye colour. They are a result of a high concentration of melanin in the iris. Brown eyes have more protection from the sun, which more than likely had evolutionary benefits.

There is more genetic variation amongst those with brown eyes than those with blue eyes.

Blue eyes

Blue eyes are the result of low concentrations of melanin. This allows more light to reflect back to wavelengths on the blue colour spectrum, making them appear blue.

According to researchers at the University of Copenhagen, a genetic mutation in one person in Europe 6-10 thousand years ago led to blue eye colour developing.

Green eyes

Green eyes are very rare and are also a result of a genetic mutation. There are low levels of melanin with green eyes but more than blue eyes. Lack of melanin in the iris causes light to scatter out and make eyes appear green.

Hazel eyes

Hazel eyes are incredibly unique and are often mistaken for green or brown eyes. Hazel eyes have the most melanin after brown eyes and usually feature a combination of colours in shades of green, brown and gold.

Summary

While eye colour is mainly determined by variations in a person's genes and usually follows an inheritance pattern, genetic variations can produce unexpected results. In addition to this, many disorders can affect eye colour, including ocular albinism and heterochromia.