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C. McMahon, S. Awua, J. Neitz, M. Neitz; Ratio of L:M Cones in Males of African Descent . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1908.
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Purpose: Among Caucasian males it has been estimated that the ratio of long (L) to middle wavelength (M) sensitive cones in the central retina averages ca. 2:1. A stochastic mechanism has been proposed to be responsible for the choice of a cone photoreceptor to express an L versus an M photopigment gene. In each cell a stable and permanent complex is hypothesized to form between a unique locus control region (LCR) 5' of the X-chromosome pigment gene array and the promoter of a randomly chosen gene in the array. Normal arrays have an L gene first (5'-most), followed by M genes, so the L gene is physically closest to the LCR. This has led to the hypothesis that the proximity of the LCR to the L gene promoter makes it more likely to bind the LCR and thus be selected for expression. The preponderance of L cones has been taken as strong evidence in favor of this hypothesis. If proximity to the LCR is a fundamental determinant of the probability of expression, then ethnic populations with the same array structure found in Caucasians should have a similar distribution and average ratio of L:M cones. We tested this prediction. Methods: L:M cone ratios were estimated using the flicker photometric electroretinogram (ERG). Subjects were African (n=13) and African-American (n=14) males, ages 15 to 43 years, with normal color vision. Each subject's X-chromosome pigment gene array was characterized by standard molecular methods. Results: All subjects had an L gene in the first position of their array. The African and African-American populations were not significantly different in L:M ratio. The range of variation in cone ratio in males of black African descent versus Caucasians was similar, however the mean ratio for subjects of African descent (1.27 L:1 M) was significantly different from the mean (1.9 L:1 M) previously estimated for Caucasians. Conclusions: These results suggest that a strong bias for expression of the first gene in the array is not a fundamental characteristic of the mechanism that determines whether a cone will be M or L. If it were, males of black African descent and Caucasians would be expected to have a similar bias in L:M ratio.
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