A Possible Cure for Color-Blindness?

Adult monkeys can see red for the first time after a gene treatment.

The inability to see red is caused by a genetic defect that affects about 1 in 12 men and 1 in 230 women.

For years scientists thought that our ability to see new types of visual input ended after adolescence. But it turns out that when researchers surgically injected a particular human gene into adult lab monkeys with this kind of color-blindness, they could recognize red for the first time. Look for yourself on this video of a monkey named Dalton picking out a red patch in an array of gray dots. (The English chemist John Dalton discovered his own colorblindness in 1798). Since most human vision disorders are partly caused by genetic defects involving the retina, the result suggests possibilities for curing a range of eye problems.

Colorblindness gets in the way of ordinary life, say researchers Maureen and Jay Neitz of the University of Washington. Severely colorblind people can’t see the difference between a rare and well-cooked steak, or pick out a red crayon from a pack. They may have trouble selecting a ripe tomato, seeing a sunburn, or performing certain jobs. Some children are mistakenly labeled as learning-disabled when they are put through color-coded tests.

Suddenly seeing red could be stressful, but according to the researchers, the monkeys who learned to see red aren’t showing signs of distress. The team concluded that seeing the color did not require new neural wiring in the monkey’s brains; they already had a blue-yellow pathway, which split into two systems, one for blue-yellow and a second for red-green. Normally, the human retina has three kinds of cone cells, each containing a different pigment that absorbs particular kinds of light. One is most sensitive to light with a short wavelength that corresponds to shades of blue, one to medium wavelengths that we see as green, and the third to longer wavelengths that, in combination with the other cones, bring us reds.

Many mammals have two cones, for example dogs, who see colors much like humans with this kind of colorblindness but see better than humans do in dim light.

Why is color-blindness so much more common in boys? The genes that produce photopigments are carried on the X chromosome, from the mother. One in 6 women can give the gene to a child. In boys, each cell carries an X chromosome from the mother, alongside a Y chromosome from the father. Girls inherit an X chromosome from both parents, so even if their mothers give them the defect, they can get correct genes from their fathers.