The molecular genetics and evolution of primate colour vision.

Until recently, the genetic basis of colour vision could only be inferred from measuring the colour vision of family groups. However, in the past few years the sites of the genes for visual pigments have been located and sequenced. The genes that specify the opsins for the rod and short-wavelength cone pigments are located on the third and seventh chromosomes, respectively. In Old World primates the genes for the middle- and long-wavelength pigments are located on the q arm of the X chromosome in a head-to-tail array. The close sequence similarity of the two genes on the X chromosome leads to a high frequency of unequal inter- and intragenic recombination leading to gene deletion or the creation of hybrid genes. In New World primates there is only a single locus on the X chromosome for a middle- to long-wavelength cone pigment. However, three alleles can occur at this locus and each codes for a slightly different cone pigment. As a result there are three types of male dichromat and three types of female dichromat and trichromat in each species. Colour vision in New World primates might be an intermediate stage between the uniform dichromacy of non-primate mammals and the uniform trichromacy of Old World primates. Alternatively, colour vision in New World primates might be an adaptation to allow a wide variety of colour-vision types within a single family group.