COLOUR BLINDNESS

inability to distinguish one or more of the three colours red, green, and blue. (Ability to see colour exists in only a few vertebrates, including, among others, man and the other primates, fish, amphibians, some reptiles, and some birds; and in bees and butterflies.) In the retina, the light-sensitive layer of tissue that lines the back and sides of the eyeball, there are, in human beings, three types of cones, the visual cells that function in the perception of colour. One type absorbs light best in wavelengths of blue-violet and another in the wavelengths of green. The third type is most sensitive to wavelengths of yellow but is also sensitive to red. Colour-blind persons may be blind to one, two, or all of the colours red, green, and blue. (Blindness to red is called protanopia; to green, deuteranopia; and to blue, tritanopia.) Red-blind persons are ordinarily unable to distinguish between red and green, while blue-blind persons cannot distinguish between blue and yellow. Green-blind persons are unable to see the green part of the spectrum. Colour blindness, which affects about 20 times as many males as females, is a sex-linked recessive characteristic. A woman must inherit the trait from both parents to be colour-blind. A colour-blind man and a woman of normal colour vision have daughters who have normal colour vision but are carriers of the traitthat is, the daughters may have colour-blind sons and daughters who are carriers. The sons of a colour-blind man and a woman with normal vision themselves have normal vision and are unable to pass the colour-blind trait on to offspring. The son of a normal man and a carrier woman may be colour-blind, and the daughter of such a union may be a carrier. Thus, colour blindness tends to skip generations.