[Molecular genetics of red-green color blindness]

Normal colour vision is trichromatic and is mediated by the blue, green and red visual pigments present in the corresponding blue, green, and red cone cells of the retina. The red and green pigment genes have evolved from an ancestral pigment gene and reside in a head-to-tail tandem array on the long arm of the X chromosome. This arrangement and a high degree of homology predispose to illegitimate recombination between the red and green pigment genes explaining the various forms and the high frequency of red-green colour vision defects.