Biodesign Scientist Suggests that Men and Women May See the World Differently

TEMPE, Ariz., July 30, 2004 —Stereotypes about the superior color sense of women may be rooted in genetics. Brian Verrelli, a researcher at the Biodesign Institute at Arizona State University, is co-collaborator on a study suggesting that natural genetic selection has provided women with a frequent ability to better discriminate between colors than men. So, literally, men and women often do see the world differently.

The results of the study by ASU’s Verrelli and Sarah Tishkoff of the University of Maryland, appear in the upcoming issue of The American Journal of Human Genetics. Their research focused on the gene that allows people to perceive the color red; a gene that is found only on the X-chromosome. They found that the gene has maintained an unusual amount of variation-about three times that of other genes.

“Normally, this degree of genetic variation is suppressed through natural selection,” said Verrelli. “In this case, nature is supporting a high degree of variation instead.”

Verrelli explains that variation in the red gene is created via the exchange of genetic material with a neighboring gene that detects green. The scientists speculate that enhanced color perception was important when women were the primary gatherers in the hunter-gatherer phase of human existence. It would have allowed them to better distinguish among fruits, foliage and insects. Therefore, nature supported the variation, despite some negative consequences to men.

Because women have two X-chromosomes, women can receive one chromosome with the typical configuration of the red vision gene while the other chromosome receives a slight variation. It is the combination of a normal and variant gene, occurring in about 40 percent of women, that may result in a broader spectrum of color vision in the red-orange range.

By contrast, men have one X-chromosome, and any variation in the single red gene that they receive reduces their ability to distinguish between red and green. This accounts for the relatively high percentage of men-8 percent-who have a color vision deficiency. It was this statistical aberration that first interested Verrelli in pursuing this research.

“Most detrimental conditions caused by a genetic variation-such as cystic fibrosis-affect a tiny fraction of one percent of the population,” Verrelli said. “The fact that the problem of color-blindness was so common suggested an important mitigating advantage,” said Verrelli.

While genetic research has traditionally focused on significant mutations in genes, the research of Verrelli and Tishkoff suggests that subtle variations may exist for explicit reasons and are worthy of attention. This research adds considerably to the knowledge base on color vision deficiency.

While the common term used is “color blind,” Verrelli notes that color vision deficiency is the more appropriate descriptor. Deficiencies are grouped into three commonly-identified conditions, but within these, individuals experience varying degrees of color deficiency.

Our perception of color comes from our ability to distinguish red, green and blue. The combination of these three colors forms the basis for all the colors humans perceive. Other forms of life have differing systems of color vision, and these distinctions may help us understand how and when life forms separated in the evolutionary process.

The researchers honed in on the advantages of subtle genetic variation with the aid of sophisticated computer analysis. Verrelli used computer technology to build evolutionary scenarios to demonstrate that this level of variation could not be explained through random biological processes. They studied the DNA of 236 randomly-selected men from geographically diverse populations, discovering that some of the variation occurred an astonishing 80 percent of the time. Men were studied because the presence of just one red vision gene allowed a way to establish parameters for the frequency of the variation.

“Until recently, we did not have powerful enough statistical modeling programs to make sense of the subtle variations,” said Verrelli. “While we’re amassing an incredible amount of genetic data, it is the accompanying new technology that allows us to make sense of the vast array of variables.”

Verrelli is a researcher in the Biodesign Institute’s Center for Evolutionary Functional Genomics, and is an assistant professor of biology in ASU’s School of Life Science. His collaborator, Tishkoff, is an assistant professor of biology at the University of Maryland.

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For images “simulating” the difference of vision go here:
http://www.biodesign.org/news/color-aug4.html