Rachelle Cocker, of Rongotea School, asks :-

What causes different eye colours?

James Borthwick, an opthalmologist, responded.

The structure which gives the eye its colour is called the iris. The iris is at the front of the eye behind the clear cornea. It has a round hole in its middle called the pupil. The main function of the iris is to change the size of the pupil to increase or decrease the amount of light entering the eye.

Within the structure of the iris lie cells called melanocytes which contain pigment called melanin. There are similar cells in our skin; if the skin has a lot of melanocytes with a lot of pigment, the skin is brown or black. If there are few melanocytes with little melanin, the skin is white. In the eye if there is a lot of melanocytes and they contain a lot of melanin, the iris appears brown, if there is little the iris appears blue. Other iris colours result from an intermediate distribution of melanocytes.

The varying distribution of melanocytes can result in different colours occurring in the same eye or sometimes people have different coloured eyes. Clumps of melanocytes can occur in the iris like freckles on the skin, giving the iris a speckled look.