Colour blindness.

(a) A pedigree for red-green colour blindness showing all possible offspring. (b) A standard eye test to screen for red-green colour blindness. People with red-green colour blindness will not see the number 74 hidden in this picture.

In daylight, about 92% of human males and over 99% of females have normal colour vision. However, problems in colour vision may result from defects in the cone pigments arising from mutations in the opsin genes. The most common is red-green colour blindness, which occurs predominantly in men (1 in 12 males, compared with 1 in 200 females). Individuals with red-green colour blindness either lack the red or green cone pigments entirely or, more commonly, have them in an abnormal form. In one form, for example, an abnormal green pigment responds to red light as well as green, making it difficult to discriminate between the two colours.

Colour blindness was first described in the scientific literature in 1794 by John Dalton, the chemist mentioned in Chapter 2 after whom units of molecular mass are named. Dalton was himself colour blind and willed that his eyes be preserved so that someday it might be possible to determine the cause of the defect. He hypothesized that it arose because his aqueous humour was filled with a blue-coloured medium of some type, but this was proven wrong after his death when one of his eyes was carefully examined. In fact, colour blindness results from a recessive mutation in one or more genes encoding the opsins. Genes encoding the red and green opsins are located very close to each other on the X chromosome, while the gene encoding the blue opsin is located on a different chromosome. In males, the presence of only one X chromosome means that a single recessive allele from the mother will result in red-green colour blindness, even though the mother herself is not colour blind.

Incidentally, in 1994, DNA testing of John Dalton's retina confirmed that he had classic red-green colour blindness. For more than 100 years after his death, colour blindness was known as Daltonism.