The Biology of Congenital Color Defects

Protanopia, deuteranopia, protanomaly, and deuter-anomaly all arise from changes in the X chromosome opsin genes. As noted previously, unequal recombination events that occur during meiosis can lead to additions and losses of genes from the X chromosome as well as to the production of novel genes (Fig. 5). Thus, a loss of the L cone opsin produces a protanopic dichromacy; loss of the M opsin yields deuteranopia. On the other hand, the anomalous trichromacies reflect the genomic presence of atypical opsin genes. In these cases, a new pigment replaces either the normal M pigment (in cases of deuteranomaly) or the normal L pigment (in protanomaly). The spectrum of

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