Colour vision deficiency, or colour blindness, is the inability to distinguish certain colours. The majority of colour blind cases are hereditary, although deficient colour vision may be acquired as a result of another eye disorder. Colour blindness affects about 8% of men, and approximately 0.5% of women¹. This is becaus the gene for colour blindness is X-linked recessive - meaning that colour blindness manifests only when there is no corresponding 'normal' colour vision gene. Since men only have one X chromosome, the chances of colour blindness showing up in men are much higher than in women, who have two X chromosomes. However, women may be carriers of the colour deficient gene, but not experience colour defective vision.

To understand defective colour vision, ‘normal’ colour vision must first be understood.

When light enters the eye, it passes through several structures before striking the retina at the back of the eye. The retina contains all the receptors sensitive to light. There are two types of light sensitive receptors in the retina - the rods and the cones. Rods are responsible for night vision, and are located in the peripheral retina. Cones are responsible for colour vision and function best under daylight conditions. Cones are also responsible for good vision and are concentrated at the fovea - the area of best vision².

There are three types of cones: red cones, blue cones and green cones. Each type of cone has a different range of light sensitivity and their stimulation in various combinations accounts for the perception of colours. In an individual with no colour defects, the cone population consists of 74% red cones, 10% green cones and 16% blue cones. Although their sensitivities overlap, each type is most sensitive to a specific portion of the visual spectrum. The stimulation of cones in various combinations accounts for the perception of colours¹.

Colour discrimination occurs through the integration of information arriving from all three types of cones. For example, the perception of yellow results from a combination of inputs from green and red cones, and relatively little input from blue cones. If all three cones are stimulated, white is perceived³.

¹ Tortora, G. J. & Grabowski, S. R. (1996). Principles of Anatomy and Physiology. Harper Collins: New York.
² Martini, F. H., Ober, W. C., Garrison, C. W., Welch, K., Hutchings, R. T. (1995) Fundamentals of Anatomy and Physiology. 3^rd ed.

Prentice Hall: New Jersey.
³ Birch, J. (1993). Diagnosis of Defective Colour Vision. Oxford University Press: Oxford.