Variety of photopigment genes underlying red-green colour vision

To understand the relationship between colour vision genotype and phenotype one needs experimental approaches that allow accurate determination of both. An important aspect of colour vision genotype is the number and ratio of genes that encode the long- and middle-wave-sensitive cone pigments. It has become standard practice to estimate these by quantitative Southern analysis, This method has many inherent problems that limit its usefulness as a quantitative measure, and its validity in estimating the number and ratio of X-linked pigment genes has never been demonstrated. Recently, a new method to determine accurately and reliably the number of long- and middle-wave genes on the X-chromosome was developed. Genotypes determined by the new method have been confirmed by independent experiments including direct sequence analysis of the genes. These results show that men with normal colour vision often have more than one long-wave gene and that the number of X-linked visual pigment genes in individuals is much greater than was previously suspected. The issue of which of the X-linked visual pigment genes are expressed in the retina is critical to understanding colour vision genotype/phenotype relationships, particularly in light of the new understanding of genotype. Since expression of any one pigment gene, sometimes in a very small number of cones, can have a significant impact on colour vision, it is important to examine levels of pigment gene expression using the most sensitive methods available. We have demonstrated that expression of as many as three or four different X-linked pigment genes in individual male retinas at visually relevant levels is common.