Genetics of color vision deficiencies.
The normal X-chromosome-linked color vision gene array is composed of a single red pigment gene followed by one or more green pigment genes. The high degree of homology between these genes predisposed them to unequal recombination, leading to gene deletions or the formation of red-green hybrid genes that explain the majority of the common red-green color vision deficiencies. Gene expression studies suggest that only the two most proximal genes of the array are expressed in the retina. The severity of the color vision defect is roughly related to the difference in absorption maxima of the photopigments encoded by the first two genes of the array. A single amino acid polymorphism (Ser180Ala) in the red pigment accounts for the subtle difference in normal color vision and influences the severity of color vision deficiency. Blue cone monochromacy is a rare disorder that involves absence of red and green cone function. It is caused either by deletion of a critical region that regulates expression of the red/green gene array, or by mutations that inactivate the red and green pigment genes. Total color blindness is another rare disease that involves complete absence of all cone function. A number of mutations in the genes encoding the cone-specific alpha- and beta-subunits of the cation channel and the alpha-subunit of transducin have been implicated in this disorder.