Below-normal ability to distinguish between colours
- Usually present from birth
- More common in males
- Usually due to an abnormal gene inherited from the mother
- Lifestyle is not a significant factor
Colour blindness is the reduced ability to tell certain colours apart. It is due to a defect in the cones, the specialized cells in the light-sensitive retina at the back of the eye. There are three types of cone cell, each of which is sensitive to blue, green, or red light. If one or more type of cell is faulty, colour blindness results. The condition is usually inherited.
What are the types?
The most common type of colour blindness, red–green colour blindness, affects far more males than females, and may take one of two forms. In one form, people are unable to distinguish between pale reds, greens, oranges, and browns. The other form makes shades of red appear dull and indistinct. Red–green colour blindness is caused by an abnormal gene carried on the X chromosome. It mainly affects men because women have a second X chromosome that usually masks the effect of the gene (see Gene disorders). However, women may pass the abnormal gene to their children.
Another, much rarer, type of colour blindness makes it difficult to distinguish between blues and yellows. This form of the condition can be inherited, but because it is not linked to the X chromosome, it affects both females and males in equal numbers. Macular degeneration and other eye disorders may cause colour blindness. The toxic effects of some drugs, including chloroquine (see Antimalarial drugs), may also cause colour blindness.
What might be done?
Colour blindness is usually noticed during routine vision testing in childhood (see Vision tests in children). It may also be detected during medical tests for jobs requiring normal colour vision, such as flying aeroplanes. The test checks your ability to see numbers in patterns of coloured dots.
Colour blindness is rarely a serious problem. Inherited forms are untreatable, but, if the condition is caused by eye disorders or drugs, the underlying cause can sometimes be treated.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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