Genes and Photopigments

Single genes specify the opsins of each of the types of cone pigment. The human opsin genes were first isolated and sequenced in 1986. As long predicted from observations about the inheritance of color vision defects, the genes for the M and L pigments are located on the q-arm of the X chromosome where they lie in a head-to-tail tandem array. Each of the X chromosome genes specifies an opsin composed of 364 amino acids, and the two are so similar in structure that the amino acid sequences for M and L pigments are 96% identical. Most individuals (at least 75% of the population) have more than one copy of the M cone pigment gene, although the functional significance of these ''extra'' genes remains debatable. The S cone opsin gene is autosomal, located on chromosome 7. It specifies a photopigment opsin containing 348 amino acids. The sequence of this gene differs from the other two enough to produce an S cone opsin that is 40-45% identical to that of the M and L cone pigments.

The close physical proximity on the X chromosome of the M and L opsin genes and their great sequence similarities makes them particularly susceptible to unequal recombination during meiosis. Such recombination events can result in the loss or the gain of complete copies of the opsin genes, or they can produce novel genes through recombination of partial sequences drawn from the original M and L genes. All of these changes will have an impact on the nature of the photopigments that get produced and hence they will alter significantly the details of color vision in that individual. For example, a loss of either the M or the L pigment gene reduces the retina so that it contains two, not three, types of cone pigment and consequently limits the individual to a dichromatic form of color vision.

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