Appliance of Science: 'Why do people have different-coloured eyes?'

Children have a great way of questioning the life around them. As we get older we often lose our sense of wonder, but children, with their natural curiosity, make great scientists. Sábha, is ten and she lives in Bray with her parents and twin sister, Lile, and she has a wonderful scientific mind. Sábha would like to know…
Appliance of Science: 'Why do people have different-coloured eyes?'
The eyes have it!
The eyes have it!

Children have a great way of questioning the life around them. As we get older we often lose our sense of wonder, but children, with their natural curiosity, make great scientists. Sábha, is ten and she lives in Bray with her parents and twin sister, Lile, and she has a wonderful scientific mind. Sábha would like to know… “Why do our eyes have colour and why do different people have different coloured eyes?”

Eye colour

The coloured part of our eye is called the iris, named after the Greek goddess for rainbows. Which is kind of fitting really, as, there are so many colour variations to the human eye.

There are two layers in the iris, the back epithelial layer and the front stroma. Both layers contains special cells called melanocytes, they make the pigment (the colour) that determines the overall colour of the iris. This pigment is called melanin and it is the same pigment that gives our skin its colour. In general, the more melanin we have, the darker our skin, or our eye colour. It is likely that eye colour is there for the same reason as skin colour, to give some protection from the Sun.

How much melanin?

The colour of our eyes depends on how much melanin we have in our melanocyte cells. What is really interesting though is that melanin doesn’t actually come in blues or greens, only in versions of brown. So how do some people have blue or green eyes? Firstly, it is more about the amount of melanin they have in their melanocyte cells. Large amounts of melanin generally result in brown eyes. A little less and eyes can appear green; less again and we get blue eyes.

In fact, in people with blue eyes, the stromal layer of the iris can be completely transparent, containing no pigment at all. So how are blue eyes blue?

Bring in the physics

Time for a bit of physics. The blue in our eyes in actually the result of blue light being reflected back out of the iris. Light passes through the transparent stroma and some of it is scattered on tiny micro particles within the layer. Blue light is most easily the one that is scattered and reflected as it has such a short wavelength. So the blue light is reflected back from the iris and the eyes appear blue. This is called the Tyndall effect and it is also explains why the sky appears blue.

Rethinking our genetics

You may have heard that the colour of our eyes is determined by our genes and that is true, but, if you were taught that it is down to something called Mendelian genetics (one copy of an eye colour gene from your mum, one from your dad) then you need to rethink things a little. It is now know that there is not one single pair of genes that control the colour of our eyes. There are at least ten different genes involved, two in particular, called OCA2 and HERC2, seem to have the biggest influence on how much melanin pigment is in the iris.

All of these things come together to explain why Sábha and her sister Lile have blue eyes, but Sábha’s are light blue and Lile’s are dark blue. It is all down to pigments, and light and scattering and reflecting and genetics. All this science comes together to enhance the true beauty of the human eye, whatever the colour.

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