Normal human colour vision is trichromatic, based on 3 types of cones that are maximally sensitive to light at approximately 420 nm (blue cones), 530 nm (green cones), and 560 nm (red cones). Neural circuits compare light absorbed by these 3 cone types to perceive those primary colours and combinations of them. Colour vision deficiencies result from genetic mutations that affect the expression of the full complement of cone photoreceptors and are classified by severity of deficiency.
Anomolous trichromacy is the mildest form. Although affected individuals express all three cones, the way the cones process the primary colours is aberrant so discriminating various colours is difficult. Anomolous trichromacy is subdivided into protanomaly (affects red cones), deuteranomaly (affects green cones) and tritanomaly (affects blue cones). Dichromacy is the next severest colour vision deficiency. Dichromats have reduced colour vision based on the use of only 2 types of cone photoreceptors. Dichromacy is subdivided into protanopia (no functional red cones), deuteranopia (no functional green cones) and tritanopia (no functional blue cones). Monochromacy is the severest form of colour vision deficiency in which colour discrimination is absent due to dysfunctional or non-functional cones. All vision is therefore mediated by rods which otherwise usually function only in night conditions (see reviews Deeb 2005, Simunovic 2010).
Tritan (blue-yellow, blue colourblindness, tritanopia; MIM:190900) deficiencies are rare (1 in 500) (Went & Pronk 1985) compared to those involving green- and red-cone deficiencies. The first report of tritan defects was in 1952 (Wright 1952). Tritan deficiencies are inherited as autosomal dominant traits (Kalmus 1955) and are a result of missense mutations in the blue-cone photopigment gene OPN1SW, leading to amino-acid substitutions in the protein sequence. Tritan defects are characterized by a selective deficiency of blue spectral sensitivity (Weitz et al. 1992). Three amino-acid substitutions which cause deficiency or loss (or both) of blue-sensitive cone photoreceptors are Gly79Arg, Ser214Pro (Weitz et al. 1992) and Pro264Ser (Weitz et al. 1992b).