The majority of inherited red-green color vision defects are caused by rearrangement and deletions of the L and M opsin genes on the X-chromosome that come about by meiotic recombination during oogenesis in females.
A dichromatic. phenotype results in males who inherit an X-chromosome in which all but one of the opsin genes have been deleted. A protanopic type defect is characterized by the absence of L opsin genes, and a deuteranopic type defect is characterized by the absence of M opsin genes
Although humans often have more than just two opsin genes on the X-chromosome, only two are typically expressed, and these determine color vision phenotype.
If meitoic recombination creates an array in which the first two genes encode pigments with identical spectral properties, a male who inherits the array will be dichromatic, either protanopic or deuteranopic.
However, a male with an array in which the first two genes encode opsins of the same class (M or L), but differ slightly in spectral sensitivities, will be an anomalous trichromat.
He will be protanomalous if the genes encode two M opsins, or deuteranomalous if they encode two L opsins. The extent of color vision loss in anomalous trichromats is determined by the degree of similarity in the spectral peaks of the two pigments.
It has been assumed that the 30 nm spectral separation between the L and M pigments was optimized during evolution, and that there is also an optimal ratio of L and M cones.
However, recent studies have shown that much smaller spectral separations between, and skewed proportions of, L and M cones still provide robust red-green color vision.
For example, red-green chromatic sensitivity was recently demonstrated to exhibit a non-linear relationship with photopigment proximity, and observers did not show a reduction in color discrimination that could be reliably classified as just outside the normal range until the L/M separation was reduced to about 12 nm or less. to know more about Color vision test
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