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Normal human colour vision involves three classes of retinal cone with maximal light sensitivity at approximately 420 nm (short wave, blue), 530 nm (medium wave, green), and 560 nm (long wave, red).

A given photoreceptor cell will contain a single type of photopigment comprising opsin protein bound to chromatophore.

Box 6.8 Red-green colour vision defects

Among north Europeans approximately 8% of males and 0.5% of females have defective colour vision ranging from mild to very severe. Dichromatic vision is a severe defect in colour vision in which individuals have no functional red cones (pro-tanopes, approximately 1% of males), green cones (deuteranopes, 1% of males), or blue cones (tritan-opes, affecting less than one in 10 000 males due to a mutation in the OPN1SW gene on chromosome 7). Less severe defects are also common, involving red-green chimera genes; these conditions are described as anomalous trichromatic disorders, either protanomalous or deuteranomalous, affecting approximately 1% and 5% of north European males, respectively. Among females, homozygotes for protan or deutan gene arrays will have defective colour vision (0.5%); 16% are expected to be heterozygous carriers, and the majority have normal vision although extreme skewing of X inactivation can lead to defective colour vision. Enhanced colour vision has been found in some females who are carriers of anomalous trichromacy due to possession of four classes of cone photoreceptor (red, green, green-like, and blue) resulting from X chromosome inactivation (Jordan and Mollon 1993).

The true absence of colour discrimination, achromatopsia, is thought to have been first described in 1777 in a report describing a subject that

. . . could never do more than guess the name of any color; yet he could distinguish white from black, or black from any light or bright color . . . He had 2 brothers in the same circumstances as to sight; and 2 brothers and sisters who, as well as his parents, had nothing of this defect' (Huddart 1777).

Congenital causes of achromatopsia include the absence of all sensitivity mediated by cone pigments (rod monochromacy) and individuals with rods and blue cones only (blue cone monochromacy). The study of the molecular genetics of blue cone mon-ochromacy proved highly informative in understanding regulation of red/green pigment genes. The condition is very rare, found in only one in 100 000 people, but Nathan and colleagues were able to determine that among 12 affected families either a single visual pigment gene was present but inactivated by a point mutation, or in about half of the cases a deletion of variable size was present 5' to the red/green pigment genes (Nathans et al. 1989). Remarkably, the common region present in all the deleted regions was found to span a highly conserved stretch of DNA which in transgenic mice revealed evidence of a locus control region present 3.1-3.7 kb 5' to the red pigment gene, essential for expression of red and green pigment genes (Wang et al. 1992).

one copy of the gene encoding red pigment followed by one or more copies of the gene encoding green pigment (Fig. 6.12) (Nathans et al. 1986; Vollrath et al. 1988).

High sequence identity has predisposed to non-allelic homologous recombination: among male Caucasians, half carry two green pigment genes, one-quarter have a single copy, and one-quarter have three or more copies. However, only the OPN1LW (red) gene and the proximal OPNTMW (green) gene are expressed in the retina, and having additional copies of the OPNTMW pigment gene does not affect colour vision (Yamaguchi et al.

1997). The other photopigment-encoding genes found in humans are RHO, encoding rhodopsin found in rod photoreceptor cells, and OPNTSW, encoding 'opsin 1 short-wave-sensitive', the photopigment found in blue cones. These genes are found at 3q21-q24 and 7q31.3-q32, respectively, and share only 40-44% identity with the red and green pigment genes.

The long-wavelength-sensitive (red) and medium-wavelength-sensitive (green) cone pigment genes are believed to have arisen relatively recently by a duplication event within the Old World monkey lineage,

Locus control region

OPN1LW Red cone photopigment

OPN1MW Green cone photopigment

Copy of OPN1MW not expressed

Locus control region

OPN1LW Red cone photopigment

OPN1MW Green cone photopigment

Copy of OPN1MW not expressed

Intergenic recombination

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