Why are some "color-blind" people able to perceive an almost normal range of colors while others see barely more than a two-toned world? The answer, says a report in the 1 November issue of Science, lies in the genetic makeup of persons suffering from a partial loss of photoreceptors.
Color blindness occurs if the eye is missing one of the three classes of cone photoreceptors, each of which corresponds to one of three colors: red, green, and blue. "As long as you have at least one from each category, you're OK," says co-author Jay Neitz, a vision researcher at the Medical College of Wisconsin, Milwaukee. "Those who don't are color blind. But the lucky ones get enough variety from their red cones to see an almost full range of color." His work focused on the most common form of the defect, called deuteranomaly, which limits the ability of 5% of U.S. males to distinguish shades of red and green.
Neitz's new work rests on a detailed molecular analysis of pigment genes that encode proteins responsible for the eye's sensitivity to various wavelengths of visible light. It provides the first experimental backing for a hypothesis, called spectral proximity, which holds that the degree of color blindness depends on the characteristics of the remaining photoreceptors--in particular, whether there is sufficient gap in the wavelengths detected by the remaining pigment to compensate for the absence of the third class of proteins. Ironically, Neitz had once rejected this idea because it failed to explain why people lacking a certain type of photoreceptor still showed normal sensitivity when exposed to that shade of light.
"Now we understand that normal variation in red pigment plays a role in the severity of color blindness," he says. "A person could have a normal variant that is slightly shifted toward green, but [the problem] is that the remaining genes are shifted [in the wavelength they detect] and are too close together" to distinguish hues.
John Mollon of the University of Cambridge, United Kingdom, who developed the latest version of the theory, says the new work offers "a very plausible explanation for the enormous variation among deuteranamolous [individuals]." The next step, he said, is to demonstrate the same results among those called protananamolous, who suffer from a less common form of color blindness involving an overabundance of red pigment genes.