THE MOLECULAR-GENETICS AND EVOLUTION OF PRIMATE COLOR-VISION

Until recently, the genetic basis of colour vision could only be infer red from measuring the colour vision of family groups. However, in the past few years the sites of the genes for visual pigments have been l ocated and sequenced. The genes that specify the opsins for the rod an d short-wavelength cone pigments are located on the third and seventh chromosomes, respectively. In Old World primates the genes for the mid dle- and long-wavelength pigments are located on the q arm of the X ch romosome 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 in ter- and intragenic recombination leading to gene deletion or the crea tion of hybrid genes. In New World primates there is only a single loc us 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 sl ightly different cone pigment. As a result there are three types of ma le dichromat and three types of female dichromat and trichromat in eac h species. Colour vision in New World primates might be an intermediat e stage between the uniform dichromacy of non-primate mammals and the uniform trichromacy of Old World primates. Alternatively, colour visio n in New World primates might be an adaptation to allow a wide variety of colour-vision types within a single family group.