Some people have a defect in the cones, the specialized cells in the retina at the back of the eye, that reduces their ability to tell certain colours apart. An inability to distinguish between red and green is the most common type of colour blindness, and this is a gender-linked genetic disorder. It is much rarer in women than it is in men.

Non-inherited causes of colour blindness include certain eye conditions, such as macular degeneration. It may also be a side effect of certain drugs, for example some antimalarial drugs. Colour blindness rarely causes serious problems but it can

Colour blindness rarely causes serious problems but it can exclude people from certain jobs where being able to distinguish between colours is essential, such as flying an aeroplane.