Brian Verrelli of Arizona State University and Sarah Tishkoff of the University of Maryland analyzed genetic data from 236 people from around the world. Specifically, they studied a gene on the X chromosome known as OPN1LW, which codes for a protein that detects visible light in the red spectrum. Exchange of material between this gene and a neighboring gene associated with green light leads to a high amount of genetic variation but can result in color blindness if the process goes awry. Among the study participants the researchers found 85 variants of the gene. That's approximately three times higher than what you see at any other random gene in the human genome, Tishkoff says. Usually it's a bad thing to have too much change in a gene, and natural selection gets rid of it. But in this case we're seeing the reverse.
The increased variation enhances the ability to discriminate between colors in the red-orange spectrum, particularly among females, because they have two copies of the X chromosome. Previous research in other primates has suggested that enhanced red vision in females allows them to better distinguish between berries and foliage when they are gathering food, Verrelli explains. If females did the gathering in prehistoric times, as many experts believe, that may explain why genetic variation promoting color sensitivity persists today. We can't explicitly test it, but the model fits, Verrelli says. The results will appear in the September issue of the American Journal of Human Genetics.