It's long been known that color blindness is caused, usually in men, by changes in the red and green opsin genes, the genes that enable humans to perceive color.
But a new study of randomly selected people from geographically diverse populations shows that normal variation in the red opsin gene may have been maintained by natural selection to give humans, especially women, a better perception of color.
In a study to be published in the September edition of The American Journal of Human Genetics, Brian Verrelli, formerly a postdoctoral fellow at the University of Maryland and now an assistant professor at Arizona State University, and Sarah Tishkoff, assistant professor of biology at the University of Maryland, found that one of the genes connected to color vision has maintained an unusual amount of genetic variation, possibly for millions of years of human evolutionary history.
Verrelli and Tishkoff studied the DNA of 236 men originating from various global populations from Africa, Europe, and Asia. They specifically looked at genetic variation of OPN1LW, the gene located on the X chromosome, which codes for an opsin protein that detects the red spectrum of light. This gene sometimes exchanges amino acids with a neighboring color vision gene, which codes for an opsin protein that detects the green spectrum of light.
The exchange of genetic material between the red and green color vision genes results in large amounts of genetic variation. However, this exchange can also sometimes go awry and result in color blindness. In fact, eight per cent of the world's men are color blind.
"We found 85 variants in this gene," Tishkoff said. "That's approximately three times higher than what you see at any other random gene in the human genome. 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. Genetic exchange, or conversion, between the red and green opsin genes has created a greater number of variations than we typically see, and natural selection is acting to maintain that variation."
Those variations may have been especially important, Verrelli and Tishkoff speculate, in a time when humans were hunter-gatherers. Enhanced color perception would have allowed women, who were traditionally gatherers, to better discriminate among colored fruits, insects and background foliage.
The X Factor
The chromosomal difference between women and men is the key to why variation of the OPN1LW gene may have different results in women and men. Women have two X-chromosomes; men have only one X-chromosome and one Y-chromosome. Because this color vision gene resides on the X-chromosome, rare detrimental changes at this gene cause color-blindness in males, whereas females are likely to have at least one good copy of the gene.
However, Verrelli and Tishkoff show that color vision changes can be beneficial too. Because females can have two different versions of this gene, but men can have only one, females may be able to perceive a broader spectrum of colors in the red/orange range. "Men and women may be literally seeing the world differently," Tishkoff said.
Subtle Change Important
The findings also suggest, says Tishkoff, that geneticists may want to look at the impact subtle changes have in natural selection, especially in disease prevention. "Biophysicists have thought that if it's not a dramatic change, it doesn't matter. But we see in this study that subtle changes can make a difference."