EDP column by Professor Ketan Dhatariya

“My son has been diagnosed with colour blindness. What does this mean?”

The eye senses 3 colours – red, blue and green. All the other colours are made up of combinations of these three ‘primary’ colours. The part of the eye that picks up light is called the retina. Within the retina are specialised structures that pick up either colour – known as the cones – or are just light sensitive and allow us to see in low light levels – these are the rods.

The cones are found mainly in the central part of the retina called the macula, where most of the light is focussed onto the retina. There are three types of cones, each sensitive to the wavelength of the different colours. The different colours are picked up by different pigments within the three different types of cones. Colour blindness occurs if these pigments are missing or abnormal. This means that the person may not be able to tell the difference between – for example red and green, because the pigment is not different. This means that most affected individuals are not colour ‘blind’, but colour ‘deficient’. Very rarely, however, there is a true loss of all colour perception and then people see in black and white.

The part of the DNA that is responsible for the pigment is found on the X chromosome. Because women have 2 X chromosomes, if only one is affected, the other chromosome can produce normal pigment for the cones. However, men have only one X chromosome, and if it is faulty, then there is no ‘back up’ and so the pigment for the cones is not produced, leading to colour blindness. Consequently, the condition occurs in 1 in 12 men and about 1 in 200 women. Most of these having problems with either red or green. These do not cause problems in everyday life, but may be important when trying to tell the difference between red and amber traffic lights.

Colour blindness is diagnosed using a set of pictures with coloured dots on it, called an Ishihara test. A number is embedded in the picture as a number of spots in a slightly different colour, and can be seen with normal colour vision, but not with a particular colour defect. It is not useful for young children who cannot tell their numbers.

There are no specific treatments for colour blindness, but occasionally, tinted glasses may help.