colour blindness

Hereditary defect of vision that reduces the ability to discriminate certain colours, usually red and green. The condition is sex-linked, affecting men more than women.

In the most common types of colour blindness there is confusion among the red-yellow-green range of colours; for example, many colour-blind observers are unable to distinguish red from yellow or yellow from green. Most cases of the condition are congenital and caused by a recessive gene variant located on the X chromosome. This implies that most women with the gene will only be carriers, while all men who inherit the variant will be affected by colour blindness. The gene defect disables one of the three colour receptors in the retina.

Lead poisoning and toxic conditions caused by excessive smoking can also lead to colour blindness. Between 2% and 6% of men and less than 1% of women are colour-blind.