GENES FOR GREEN: Could gene therapy "cure" male monkeys’ color blindness? To find out, biologists developed a way to incorporate the gene for a human photopigment into a small virus and injected viral particles into monkeys' eyes. Image: iStockphoto
There is ample evidence that men and women think, express themselves and even experience emotions differently (for more details, read on through this issue). But in the area of sensory perception, psychologists are hard-pressed to identify major discrepancies. By and large, the way the two genders experience the sounds, sights and smells of life is quite similar. The most striking exception may be found, at least for some, in the perception of colors.
Seeing in color is a complex process, as you may remember from your school days. It starts with the delicate lining of the eyes, a structure called the retina. Retinal tissue contains light-sensitive cells that absorb wavelengths in the visible spectrum and convert them into electrical signals. The brain interprets this information as the riot of colors we consciously experience. The retinal cells called cones come in three varieties. The S-type cone is maximally sensitive to light in the short-wavelength (blue) part of the visible spectrum, the M-type cone responds best to medium wavelengths, and the L-type to long, reddish wavelengths. People with normal color vision are known as trichromats because they possess these three kinds of photosensitive cone cells.
About 8 percent of men, but fewer than 1 percent of women, have impoverished color vision, typically because they lack the gene for either the L- or the M-type photopigment. While their vision is normal in every other way, they suffer from what is often called red-green color blindness. Depending on the specific genetic omission involved, such people—who are known as dichromats because they have only two types of cone cells—are unable to distinguish between violet, lavender and purple or between red, orange, yellow and green.
It’s not a tremendous handicap, but it can make traffic lights—especially horizontal ones—as well as warning lights that flash either yellow or red hard to decode. And a lack of sensitivity to reddish hues makes it almost impossible for a dichromat to detect the onset of sunburn. (The photographs on the opposite page show the sickly-looking hue of skin as seen through color-blind eyes.)
The reason color blindness is so much more common in boys and men is that the two genes for the L- and M-type photopigments—the substances in cone cells that absorb light—are carried on the X chromosome. A girl who inherits one defective copy of such a gene from her parents has a backup on her other X chromosome. Because men have only one X chromosome (their paired sex chromosome is a Y), they lose out. Interestingly, also thanks to the vagaries of genetics, some women are endowed with four kinds of photosensitive cones instead of the standard three. Theoretically, these so-called tetrachromats can identify subtleties of shading that are indistinguishable to the rest of us; however, this phenomenon has been hard to confirm experimentally.
Unlike humans, most mammals possess just two kinds of retinal cones. Thus, mice, cats and dogs see the world much the way a red-green color-blind person does, making them ideal experimental subjects. A few years ago scientists at the Johns Hopkins School of Medicine inserted the gene for the human L-type photopigment into mice. After several generations of breeding, the mice responded to the extra hue information. They had changed from dichromats to trichromats—a remarkable feat of bioengineering. The experiment also showed that mouse brains are flexible enough to receive and make use of the additional wavelength information.
An even more ambitious experiment, extending over a decade, came recently to fruition. It was conducted by the husband-and-wife team of Jay Neitz and Maureen Neitz, both professors at the University of Washington School of Medicine, and their collaborators. The work involved squirrel monkeys, a species indigenous to Central and South America. Among these primates, most females are trichromats, but the males are dichromats, possessing only the S- and M-type photopigments. Accordingly, it is the females that lead troops of monkeys to search for ripe fruits among the foliage, a quest that requires superior color discrimination skills.