Different coloured eyes: the beauty of heterochromia

By Becca Smithers

Have you ever spotted that some people, cats, dogs, and other animals, have eyes that are different colours? Their left eye may be blue, and their right eye may be brown. How is it possible for an individual to have two eye colours?

How do we get eye colour?

The iris is the area of the eye that has the colour. Your eye colour is decided by your genes and the combination of alleles you inherit from your parents. Alleles are variations of one gene, for example different alleles of eye colour are blue, brown, and green. These alleles code for different levels of a pigment called melanin to be produced in the iris. Higher levels of melanin will result in darker eyes.

As with most alleles, some are dominant over others. Brown eye colour is dominant over blue, so if you have a blue eyed mother and a brown eyed father you will most likely have brown eyes. It is possible for brown eyed parents to have blue eyed children, because the parents may both carry the allele for blue eyes and pass them to their child.

Heterochromia iridium

Cat with complete heterochromia

Cat with complete heterochromia

Heterochromia iridium means different coloured irises. This is where someone will have one eye of one colour, and one eye of another colour or a mixture of colours. It is caused by a genetic mutation where too much or too little eye colour pigment is produced in one iris. This alters the colour of the whole iris, or sometimes just a portion of it, and the individual has heterochromatic eyes. You may see that some people and animals have a segment of their iris which is a different colour, or perhaps a ring around their pupil; these individuals have partial heterochromia. Complete heterochromia is having two completely different coloured eyes.

Heterochromia can be acquired, it is not just inherited, for example, actress Mila Kunis has acquired heterochromia. She was blind in one eye for years due to an inflamed iris that caused a cataract and changed the colour of one eye. She now has one brown and one green eye. It is often believed that David Bowie had acquired heterochromia, but this is not true. The difference in his eye colour is not due to change in pigment levels, but a result from when Bowie got into a fight with a friend at age 15, and the friend’s fingernail cut Bowie’s left eye. His left pupil permanently dilated, which gives the impression that his eyes are different colours, when in fact they are both blue.

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Posted in Biology