Why do people have different eye colours? By Ashley Loeb

Do you ever wonder why people’s eyes are so different? Why are there so many colours? Eye colours have been a mystery to many scientists over the years. Some say it is because of genetics, others say it is merely random. If these questions has ever popped into your mind at one point or another, here you can read everything you want to know about it.

This is an x-ray of an eye. The normal eye colour is probably blue or green with brown.

What is Melanin and Pigment?

Melanin is a Pigment found in the front of your eye. Pigment is a natural colouring. Eye colours come from different amounts of pigment. For example, lack of pigment would result in blue eyes, because they are very light. Some pigment results in green eyes, as they are slightly darker, and lots of pigment results in brown eyes. If your eyes are blue-green or hazel, don’t worry, because there is an explanation for that too. Blue-green eyes have the amount of pigment between green and blue eyes, because they are darker than blue but lighter than green. Hazel eyes have the amount of pigment between green and brown eyes, because they are lighter than brown but darker than green. Although the most common eye colours are brown and blue, other rarer colours are grey, green, red, and amber. “Red eyes do exist,” Says Eye Doctors of Washington, “Red eyes are often called pink. Picture white bunnies with pink eyes. What you’re actually seeing in these rabbits and in albinos is the blood vessels behind the iris. Because there is so little melanin in the eyes, there is nothing to conceal the blood vessels hard at work.” Wouldn’t red eyes be a little creepy? Although weird, this explanation makes sense. If your eyes have no pigment or melanin, wouldn’t they just be clear? And if they’re clear, wouldn’t it make sense that the only thing visible are the blood vessels?

Why do we have eye “Patches”?

Sometimes our eyes have faint patches around the pupil. These patches are different to the colour of the actual eye. On a blue eye, brown patches are the most common, and on a green eye, a brown patch is common too. Many people who have lighter eye colours usually have patches of brown around the iris. Patches are pretty common and sometimes can cause your eyes to look completely different. This is one the reason why some people have two different eye colours. These little patches are caused by the amount of melanin produced in the iris. This means that the more melanin and pigment in the eye, the darker the colour.

Why do we have separate eye colours?

One explanation for our eye colours is heterochromia. “Heterochromia means "different (hetero-) colours (-chromia). Usually the term is used to describe the condition where a person has different coloured eyes — one blue eye and one green eye, for example,” says all about vision. Heterochromia is a mostly genetic disease that changes the amount of melanin in one’s eyes. There are three different types of heterochromia. The first type is called complete heterochromia. This means that each eye has it’s own colour. The second type is called sectoral heterochromia. This means that there is a tiny bit of contrasting colour in the iris. The different colour is usually near the pupil. The third type is called central heterochromia. This means that there is a different colour radiating out from the center of the eye. Black eyes are actually non-existent, they are just brown eyes that have lots of pigment and look black under certain lighting. According to Susan J. Gross, co-director of the Division of Reproductive Genetics at the Montefiore Medical Center, “Eye colour is a manifestation of the pigment that is present in the iris. Brown eyes are rich in melanin deposits, and blue eyes indicate a lack of melanin. The melanocytes of the iris rest in a richly innervated pseudo syncytium, which is necessary to maintain eye colour.” This means that Although heterochromia is a disease, (though not fatal), it is one of the most beautiful in the world.

Complete Heterochromia
Central and sectoral heterochromia.

What are the possibilities of heterochromia?

The possibility of having heterochromia is quite slim. In fact, only 1 percent of the human population has it. Heterochromia also occurs in animals. Have you ever seen a photo of a little puppy with one brown and one blue eye? Aren’t they the cutest? Well, just like humans, animals can be affected too. Heterochromia appears most in cats and is actually pretty common. In fact, about 60-70 percent of the cat population has heterochromia. In dogs, however, this disease is quite uncommon. Only about 3.5 percent of all dogs have it. In this 3.5 percent, most of the dogs that have a merle coat have heterochromia. “Also, eye colours don't come out as a blend of the parents' colours, as in mixing paint. Each parent has two pairs of genes on each chromosome. So multiple possibilities exist, depending on how the ‘Wheel of Fortune’ spins,” Says all about vision. Pigment is also found in other parts of the body, so this shows that the colour of the eyes can also depend on skin and hair colour.

These animals all have complete heterochromia, and these dogs all have merle coats.

“So eye colours are based on a lot of things?” You might say. This is true, they are not just based on genes, and they are definitely not random. All of this information sums up to one thing: Pigment. So the next time you look yourself in the mirror and grow jealous of the girl next door with beautiful heterochromia, look at yourself one more time, and be proud of your unique eyes.



Created with images by tolomea - "Eye" • luckylynda74 - "HPIM0004_edited" • luckylynda74 - "HPIM0054" • Look Into My Eyes - "gorgeous brown eye" • Ömer Ünlü - "BLUE" • Look Into My Eyes - "blue blue blue" • ericmay - "Amber Eyes" • diastème (Sarah Giboni) - "5775" • luckylynda74 - "HPIM0155" • Look Into My Eyes - "Green eye"

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