Daltonism

... is an alternative name for a particular form of colourblindness, scientifically known as deuteranopia. Sufferers are unable to distinguish between colours in the green–yellow–red section of the spectrum. Deuteranopia occurs in about 1% of males, but is rare in females.

Another relatively common form of colourblindness, known as protanopia, similarly occurs in about 1% of males but is rare in females; and also like deuteranopia, it results in an inability to distinguish between colours in the green–yellow–red section of the spectrum.

Deuteranopia is caused by a lack of the green cones for medium–wavelength cones, while protanopia is caused by a lack of the red cones for long–wavelength sensitive retinal cones. I am slightly confused by the double references to cones, but this is what it says on Wikipedia.

The commonest of all forms of colourblindness, affecting about 6% of males and 0.4% of females, is known as deuteranomaly. Sufferers have a mutated form of the medium–wavelength (green) pigment, resulting in a reduction in sensitivity to the green area of the spectrum. In poor light, these individuals see green objects as black.

A fourth form of colour blindness is known as protanomaly. This is similar to deuteranomaly, but results in a reduction of sensitivity to red light. It has a similar incidence to the first two types – affecting about 1% of males and 0.04% of females.

Deuteranopia is known as Daltonism after the English scientist John Dalton, who was a sufferer. (The diagnosis was confirmed in 1995 by DNA analysis of his preserved eyeball.)