Eye genes

Your parents did not choose the colour of your eyes, but they certainly are responsible for it.  Eye colour is a trait (a feature or characteristic) that is passed on, or inherited, from your parents. 

It was once thought that everyone received one eye colour gene from each of their parents, with one of those genes being dominant over the other. Researchers have now discovered that more than 150 genes influence eye colour.

Eye colour is found in the iris of the eye. The iris helps control the amount of light entering the eye by opening and closing the pupil. The colour of the iris is dependant on how much melanin is stored in its front layers. The amount and type of this natural skin pigment is responsible for skin, hair and eye colour in both people and animals. 

Specialized skin cells called melanocytes produce melanin, which is stored in melanosomes. Everyone has roughly the same number of melanocytes, but the amount of melanin within melanosomes and the number of melanosomes within melanocytes both vary from person to person.

Genes passed down from your parents are involved in the production, transportation and storage of melanin. And it is the amount of melanin stored in the melanosomes that makes our eyes appear the colour they do.

Blue eyes are the result of a small amount of melanin found within a small number of melanosomes.

Green eyes are the result of a moderate amount of melanin in a moderate number of melanosomes.

Brown eyes are the result of a high amount of melanin stored within many melanosomes.

How do we see colour?

When light hits an object that object absorbs some of that light and reflects the rest.  It is that reflected light that enters the eye through its outermost layer, the cornea. The cornea bends that light to the pupil which regulates how much of it will hit the lens. The lens then focuses the light on the retina, a layer of nerve cells at the back of the eye.

The retina contains two different photoreceptors, those cells that detect and respond to light. Rods react when you are in dim light while cones respond in brighter environments. The human eye contains about 6 million cones, and 110 million rods.

Each type of cone is sensitive to a different wavelength of visible light and contains colour detecting molecules called photo pigments (typically red, green and blue).  

In daylight the light reflected off a lemon would activate red and green cones which would send a signal along the optic nerve to the brain.  The brain processes the number of cones that were activated and how strong each signal is. Once it has made its calculations a colour is seen, in this case, yellow.

If that same lemon is seen in a darker room, only the eyes’ rods would be activated meaning shades of grey would be seen rather than colour.

Scientists estimate that humans can distinguish up to 10 million colors.

Myth busted: Dogs can actually see colour, just not as we do. A dog’s eye has only two types of cones making its perception of colour similar to that of a person with red-green colour-blindness. 

Did you know? Males are more likely to be colour blind than females. The gene responsible for colour-blindness is on the same chromosome (made up of two parts, X or Y) that determines the sex of a child. Males have X and Y chromosomes and females have two X chromosomes. Red-green colour-blindness is only found on the X chromosome. For a male to be colour blind the colour blindness ‘gene’ only has to appear on his X chromosome. For a female to be colour blind it must be present on both of her X chromosomes.

One eye with two colours, Heterokromi

In rare cases a person may have two different coloured eyes (heterochromia iridium) or different colours within a single iris (heterochromia iridis). 

Article originally published in Brainspace Magazine Spring 2021

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