Colour blindness

From Ganfyd

Homo sapiens has three varieties of colour-sensitive cells - cones - in the retina (other species have none, two, three or in South America, four). These have arisen by reduplication of the gene for the protein which holds a stressed Vitamin A residue ready to be altered by a photon and its subsequent mutation leading to different peak sensitivity wavelengths for each protein. Each cone cell turns on one of these genes, randomly, and thus populations of cones with different sensitivities form in the retina.

If one of those genes is an allele for an ineffective version of the protein then each cone expressing that gene will fail to work, and poor discrimination of light of the wavelength they would otherwise be tuned to will result.

The degree of sensitivity is variable and the maximum sensitivity is only required for certain critical tasks - recognising faint lights at sea, wiring complex coloured looms into electronic equipment and the like. The type of colour blindness can be tested with Ishihara plates but a Lantern test is required for some purposes. People who fail the Ishihara plates may well pass the lantern test.

The genes are carried on the X chromosome, and therefore this is a male problem, in Homo sapiens.