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Kids News explainer: What makes eyes different colours?

Donna Coutts, May 19, 2020 8:45AM Kids News

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Eye colour can change for about the first three years of a child’s life. media_cameraEye colour can change for about the first three years of a child’s life.

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What colour eyes do you have? If the answer is brown, you’re in the majority: somewhere between 60-90 per cent of the more than 7.5 billion people on Earth have brown eyes too.

In fact, it’s thought that 10,000 years ago, every human had brown eyes.

Why do most people have brown eyes but some have other colours? What makes eye colour? Kids News researched eye colour and this is what we learned.

WHAT GIVES EYES THEIR COLOUR?
The coloured part of the eye is a ring that opens and closes called the iris.

The job of the iris is to control how much light goes into the eye through the pupil* to the retina* at the back of the eye.

The amount of pigment* called melanin in your irises partly determines what colour your eyes are.

The more melanin, the darker the iris will be.

Brown eyes have a lot of melanin in them. Hazel eyes have less pigment, with more around the edges than towards the centre.

Other coloured eyes, such as blue, green and grey, have less melanin pigment in them, but they don’t have any blue, green or grey pigment in them. They just have less brown colour and the lighter iris reflects the light differently to give the appearance of these other colours.

Blue, then grey then green eyes are the next most common colours (in that order), with perhaps only 2 per cent of the world’s population having green eyes.

Amber eyes — copper or yellowy — are very rare. Like brown eyes, amber eyes have a lot of melanin in the front layer of the iris. But amber, hazel and green eyes also have another type of pigment called pheomelanin that helps make the eye colour.

Eye. Eyelashes. Pupil. Iris. media_cameraDifferences in eye colour are because of differences in the amount of melanin in the iris.

DOES EYE COLOUR DO ANYTHING?
As well as making eye colour, the pigment melanin helps protect the eyes from the sun’s UV rays. Darker eyes, with more melanin, are less sensitive to sunlight than lighter eyes.

WHY DON’T WE ALL HAVE THE SAME EYE COLOUR?
Scientists believe that early humans more than 10,000 years ago all had brown eyes.

At some point a gene* mutation — a change in the way a gene works — turned off the melanin pigment in the eye so that some of the humans began to be born with paler eyes such as hazel, blue, green or grey.

Eye. Eyelashes. Pupil. Iris. media_cameraBlue iris and pupil of a human eye. Scientists believe a change to the way a gene worked resulted in non-brown eyes in some humans thousands of years ago.

DO WE KNOW WHAT COLOUR EYES A BABY WILL HAVE?
Even when your parents or grandparents were at school, scientists thought eye colour was inherited through a single gene from each parent. Science teachers used to teach students to predict eye colour with a simple chart. That science was wrong!

Scientists now know that how eye colour is inherited is much more complex and involves more than just one gene. This makes it hard to predict what colour eyes a baby will have.

Eye colour is not like paint, so a baby doesn’t just get a mix of its parents’ eye colours.

Scientists do know, however, that if both parents have brown eyes, there’s a good chance a baby will have brown eyes. And if both parents have blue eyes, there’s a good chance a baby will have blue eyes.

Cute baby with beautiful blue eyes on the white bed media_cameraSome babies are born with pale or almost colourless eyes and the eventual colour develops properly over the first three years of life.

CAN EYE COLOUR CHANGE?
Some babies are born with pale blue or almost colourless eyes but a different colour develops over the first three years of life.

Over the lifespan, colour can change gradually. Sudden changes in eye colour could be a sign of ill health that should be investigated by a doctor.

glaucoma media_cameraA doctor or optometrist should examine your eyes if they suddenly change colour.

CAN YOU HAVE TWO DIFFERENT COLOURED EYES?
Occasionally you may see a dog, horse, cow, sheep, rabbit or cat that has one brown eye and one blue eye. Almost always, these animals have unusually patterned coats, as the genes for coat colour and eye colour are closely connected. In horses, this coat patterning is called piebald, pinto or paint.

Very rarely, humans have two different coloured eyes and often scientists do not know why this has happened.

GLOSSARY

  • pupil: black hole in the iris of the eye
  • retina: thin layer at the back of the eye that receives light and then sends information from to the brain
  • pigment: colour
  • gene: the way information about how a person will look or grow is passed from parent to child

EXTRA READING

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Why is our blood red?

Why is the sky blue?

Why do leaves change colour and fall in autumn?

Hand washing: how does soap work?

QUICK QUIZ

  1. What is the black spot in the middle of the coloured iris called?
  2. What is the name of the pigment that makes eyes brown?
  3. What eye colour did every early human have?
  4. What should you do if your eyes change colour suddenly?
  5. What is a piebald horse?

LISTEN TO THIS STORY

CLASSROOM ACTIVITIES
1. Fascinating Facts
Think about all of the things that you have learned about eye colour in today’s story. What are your top five most interesting or surprising facts from the story? Rank them in order from 1 (most interesting or surprising) to 5. Next to each fact, write reasons why you chose this fact, what is so interesting/surprising about it and one question that you would like to ask a scientist or specialist to find out more about this. See if you can find the answer to each question!

Time: allow 30 minutes to complete this activity
Curriculum Links: English, Science, Health and Physical Education

2. Extension
Design and create a model using things that you can find around your house that will help someone else understand how our eyes work.

Time: allow at least 40 minutes to complete this activity
Curriculum Links: English, Science, Health and Physical Education, Design and Technologies

VCOP ACTIVITY
Up-Level It
Scan through the article and see if you can locate three words that you consider to be basic, or low level. Words we use all the time and they can be replaced by more sophisticated words, words like good and said are examples of overused words.

Once you have found them, see if you can up-level them. Think of synonyms you could use instead of these basic words, but make sure they still fit into the context of the article.

Re-read the article with your new words.

Did it make it better?

Why/Why not?

HAVE YOUR SAY: What topic would you like Kids News to research?
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