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J Carroll, J Neitz, M Neitz; Localization of the Mechanism Which Makes a Cone Become L or M . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2956.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The biological mechanisms responsible for determining the identity of a cone as L vs. M are not known. However, the existence of large individual variation in L:M cone ratio provides a means to help differentiate between two competing hypotheses. The first hypothesis is that the L and M cones are separate cell types and their identity is independent of the cone pigment expressed. Under this theory, during development, before the cones express opsins they become determined to be L or M. Later the appropriate opsin is expressed based on the pre-determined identity. The second hypothesis is that the identity of a cone as L vs. M is determined only by the opsin that is expressed in it. In its simplest version, the second model predicts that L:M ratio is an X-linked trait. To test this, we obtained estimates of L:M ratio in brothers. Methods: We recently developed a method that combines genetics and ERG flicker-photometry to measure relative ratios of L to M cones in humans (Carroll et al., 2000 JOSA A 17:499). Complete spectral sensitivity functions were obtained using ERG flicker photometry for 20 pairs of brothers (26 individuals). The amino acid sequences deduced from the L gene sequences determined for each subject were used to identify individualized L pigment spectra. ERG spectral luminosity functions were best fit to a relative weighting of each person's individualized L and an M pigment template, giving an L:M cone proportion estimate. Results: Each male inherits only one of his mother's two X-chromosome L/M gene arrays. Brothers who shared the same L/M gene array differed in L:M ratio on average by about 5% L, which is similar to the differences observed for repeated measures on a single subject. In contrast, brothers who inherited different arrays differed in L:M ratio on average by 12% L, which is not statistically different from the results for randomly chosen pairs of unrelated males. Conclusion: Variation in the ratio of L to M cones in the human retina appears to be linked to the L/M gene array on the X-chromosome. This result is predicted from the hypothesis that a stochastic mechanism, associated with the L and M gene locus, operates to randomly switch on expression of one gene from the array to determine the identity of a cone as either L or M.
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