Figure 3.

The array of opsin genes on the q arm of the X-chromosome. Exons (the coding regions of the genes) are shown as narrow bars of saturated color, and introns are shown as less saturated. Typically there is only one copy of the gene for the long-wave pigment, but there may be more than one copy of the middle-wave gene. Interdigitated with these opsin genes are (truncated) copies of a gene “TEX28,” expressed in the testis (30), and from this we can infer the possible nature of the original duplication that gave rise to distinct long- and middle-wave genes. Upstream of the array is a LCR. The hypothesis of Wang and colleagues (2) is that the LCR interacts with the promoter region of just one of the opsin genes (alternative couplings are shown in blue), and this alone determines whether the cone is long-wave or middle-wave.