2.1. Color vision deficiency

Several kinds of genetic changes involving the OPN1MW gene cause red-green color vision defects, a form of color vision deficiency that makes it difficult or impossible to distinguish between shades of red, yellow, and green. Most red-green color vision defects result from structural rearrangements involving the OPN1LW and OPN1MW genes. Because these genes are so similar, they occasionally swap genetic material when the genes are being passed from parent to child. This swapping, called recombination, can ultimately delete genetic material from one or both genes or lead to the formation of a hybrid pigment gene that contains part of the OPN1MW gene and part of the OPN1LW gene.

Less commonly, red-green color vision defects can result from an OPN1MW gene mutation that changes a single protein building block (an amino acid) in the middle-wave-sensitive photopigment. This mutation replaces the amino acid cysteine with the amino acid arginine at position 203 (written as Cys203Arg or C203R).

When OPN1MW gene mutations lead to completely nonfunctional M cones, color vision depends entirely on the other two types of cones. The specific type of red-green color vision deficiency that results from a total loss of M cone function is called deuteranopia. A less severe red-green color vision defect called deuteranomaly occurs when a partially functional hybrid pigment gene replaces the normal OPN1MW gene. The photopigments made from these hybrid genes usually have abnormal visual properties that impair red-green color vision.

A rarer form of color vision deficiency, blue cone monochromacy, severely reduces sharpness of vision (visual acuity) and affects the ability to perceive most colors. This condition also includes other vision problems that are not typically found with red-green color vision defects. Blue cone monochromacy occurs when genetic changes prevent the opsin pigments produced from both the OPN1MW and OPN1LW genes from functioning normally. In some cases, the condition is caused by a deletion of the LCR, which normally controls the activity of the OPN1MW and OPN1LW genes. A loss of the LCR prevents the production of pigments from both genes. As a result, people with this condition have only functional cones with short-wavelength-sensitive photopigment (S cones), which leads to reduced visual acuity and poor color vision. The cone abnormalities also underlie the other vision problems in people with blue cone monochromacy.