The L:M cone ratio in males of African descent with normal color vision

Among Caucasian males with normal color vision, long-wavelength-sensitive (L) cones outnumber middle-wavelength-sensitive (M) cones by nearly three to one, on average, and the L and the M cone opsin genes are arrayed on the X-chromosome with the L opsin gene being closest to an upstream enhancer element termed the locus control region (LCR). Interaction between an opsin gene promoter and the LCR is required to mediate normal opsin gene expression, and the relative proximity of the L opsin gene promoter (4,000 base pairs for L compared to 40,000 base pairs for the M opsin gene promoter) has been proposed to endow the L gene with the advantage in competing for interaction with the LCR, thereby accounting for the nearly 3:1 ratio of L: M cones. This proximal advantage hypothesis predicts that the L: M cone ratio will be similar among populations that share the same X-chromosome opsin gene array organization. Here, we tested this hypothesis by examining a sample of males of African descent and found them to have a significantly different average L: M ratio compared to Caucasian males, even though their X-chromosome opsin gene arrays were indistinguishable from arrays in males of Caucasian descent. How these observations might be reconciled is discussed.