ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
Young girl taking a colour-blindness test. People with normal vision see the numbers 2 and 6, whereas those with red-green deficiencies see only a random pattern of dots. Colour blindness usually occurs as an inability to distinguish between red and green, although the confusion may arise between blue and yellow. The condition is usually caused by a defect in one of the three groups of colour-sensitive cone cells in the retina, and is untreatable. Colour blindness is more common in men than women and is inherited through both parents. Red-green colour blindness affects approximately one person in thirty.
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