Colour blindness

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The human eye has three types of light-detecting cone cells, which contain pigments sensitive to different wavelengths of light: long (red), medium (green) and short (blue). The differences in the overlapping signals from all three are used by the brain to perceive colour.

Colour blindness occurs when one or more of the cone cells do not work properly. The most common form (which accounts for over 95 per cent of cases) affects perception of light at longer wavelengths, leading to diffi culty distinguishing between red and green. Yellow-blue colour blindness occurs occasionally, and a few people can’t see any colour at all.

Colour blindness is much more common in men than in women. This is because many of the genes involved in colour vision are found on the X chromosome. For a male (46, XY) a defect on his only X chromosome is sufficient to cause colour blindness, whereas for a female (46, XX) both X chromosomes must be defective – this latter scenario occurs much less frequently.

Research has shown that individuals unable to use colour to differentiate between objects develop superior visual skills with regard to identifying texture and shape. This enables them to see ‘through’ colour camoufl age in a way that people with normal three-colour vision cannot.