This is not blindness in any sense of the word and indeed some colour-blind individuals are unaware of any problem until their colour vision is tested. Of the male population, 8% suffers from some form of congenital colour blindness. This is usually in the form of "red-green blindness". Inheritance of this type of defect is sex-linked so that unaffected female carriers pass the gene to 50% of their sons. The screening of school children for colour blindness is now widely practiced because of the occupational implications. The Ishihara test is the simplest and the best test for congenital colour blindness. Occupations that entail the reading of coloured warning lights or the matching of colours usually demand some form of colour vision test on entry. It is an advantage to the child to be aware of any defect during the early years of schooling.
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