Color: Genetics of

Genetic Basis of Human Color Vision

Cone photopigments in the human retina have two components—a protein component (opsin) and a chromophore (11-cis-retinal, a vitamin A derivative). Each of the cone photopigments has different spectral sensitivities; that is, they vary in the efficiency with which they absorb the different wavelengths of light. These cone photopigments differ only in their opsin component, and a different gene encodes for each of the opsin molecules. The gene encoding the S opsin is located on chromosome 7. The genes encoding L and M opsins are located on the X chromosome, in a head-to-tail tandem array (i.e., they lay right next to each other). This arrangement is the primary reason there is variation in these genes in humans. Just adjacent to this array is an important genetic regulatory element called the locus control region (LCR). The LCR is required for any of the genes in the L/M array to be expressed. The L and M opsin genes are about 96% identical, though they show only about 43% identity with the S opsin gene; many of these genetic differences are responsible for the different spectral properties of the [Page 250]photopigments. Mutations in these opsin genes result in differences in the structure of the associated opsin. This alters the functional properties of the photopigment; for example, the sensitivity of the photopigment to different wavelengths of light might be shifted, or the photopigment could be altogether nonfunctional. The functional consequences of genetic variation are discussed later.

The location of the L and M genes on the X chromosome means that color vision defects associated with mutations in these genes are inherited in an X-linked recessive pattern. These defects are commonly called “red-green” color vision defects. Females have two X chromosomes, whereas males have only one; therefore, in males, one mutated copy of the X chromosome is sufficient to cause a red-green deficiency, whereas in females the mutation must be present in both copies of the X chromosome. Thus, males are about 25 times more likely to have red-green defects than are females; however, nearly 15% of women will be carriers of a red-green color vision defect. Interestingly, there is rarely any behavioral indication of a female's carrier status. Thus, these red-green defects might pass through many generations without any family member exhibiting a defect.

In contrast, the autosomal (not sex-linked) location of the gene encoding the S opsin means that color vision defects associated with mutations in the S-opsin gene (referred to as “blue-yellow” color vision defects) are inherited in an autosomal dominant fashion. As a result of their autosomal location, humans normally have two copies of each of these genes; yet, a defect in a single S pigment gene is sufficient to cause a blue-yellow defect—there are no carriers. Thus, blue-yellow defects are equally prevalent in males and females, though they are much more rare than the red-green defects in general.

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