There are several forms of colour blindness, or colour vision deficiency. It's usually an inherited genetic problem.

By far the most common form is red/green colour blindness, which affects about one in ten men. There's also blue/yellow blindness and other types. Only about one in 200 women is affected.

How it's inherited

The faulty gene for red/green blindness is carried on the X chromosome.

Women have two X chromosomes, while men have one X and one Y. Women can have the faulty gene on one X chromosome and a normal gene on the other.

As red/green colour blindness is a recessive disorder, the normal gene dominates the recessive (or weak) faulty gene, so these women have normal colour vision. They don't usually have the problem, but do carry the faulty gene and can pass it on to their children.

Men with a faulty X chromosome will have the condition because they don't have a spare healthy X chromosome.

Men pass on their X chromosome to their daughters and their Y chromosome to their sons. So a man with red/green colour blindness will pass the faulty gene to his daughters. This means you're a carrier of the condition. Any brothers of yours will be neither a carrier nor affected.

A woman who carries the colour blindness gene has a 50 per cent chance of passing it on to her children, because she passes on only one of her X chromosomes to each child.

This means your sons have a 50 per cent chance of inheriting the gene from you. A son who inherits the faulty gene will have the condition. Any daughters also have a 50 per cent chance of inheriting the faulty gene. But as they'll also inherit a healthy X chromosome from their father they'll only be carriers.

The very rare exception is daughters in a family where the mother carries the condition and the father has the condition. Here, there's a 50 per cent chance that girls will inherit two faulty genes, one from each parent, and develop colour blindness.

What it means

Being colour blind rarely means a person can't see the colours at all. Instead, they have trouble differentiating between red and green colours of a similar tone. They may be able to tell a bright apple green from a postbox red, while muddling more similar shades. The degree of colour blindness varies.

The condition rarely causes any significant disability and makes little difference to the lives of most people. However, it can be very frustrating and may stop the person from doing certain jobs (it can be difficult to get a pilot's licence, for example).

Other complaints include problems with maps (especially index colours), hazard-warning lights and light-emitting diodes, colour-coordinating clothes and paint, carrying out chemical experiments, detecting sunburn or working out how well cooked meat is.

Testing

A simple test for most colour blindness is known as the Ishihara test. Talk to your doctor or optician.

This article was last medically reviewed by Dr Trisha Macnair in December 2007