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How Gene Mutations Cause Colorblindness

By Kate Ruder

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The adaptive optics technology compensates for imperfections. Top: Human retina viewed with adaptive optics (right) and without. Bottom: Jupiter’s moon Io with adaptive optics (right) and without.
Using new optics, scientists have finally seen what the back of the eye looks like in people who are colorblind. They used the same technology used by astronomers to view the night sky.

It is well known that people who are colorblind have mutations in their genes that cause a loss of either red or green cones, and they therefore have a hard time distinguishing between colors. There are three kinds of cones in the human eye: red, green, and blue.

Now researchers have discovered that some people with the gene mutation that causes colorblindness lose an entire set of “color” cones with no detriment to the acuity of their vision overall. Their retinas look like Swiss cheese with black holes where red or green cones should be.

“No one has been able to tie the genetics to the mechanism of disease because no one could see the actual number of cones before,” says Joseph Carroll, who led the research at the University of Rochester in New York.

The new imaging technology may help researchers study more serious eye diseases such as glaucoma because ophthalmologists could actually see what happens to the eye in these disorders.

Man having his retina tested. The red lines show the path of light through the adaptive optics system to the human eye.
The military and astronomers originally developed the technology, called adaptive optics, for telescopes to capture a clearer view of the sky.

In astronomy, the adaptive optics technology compensates for turbulence in the atmosphere; in ophthalmology, it compensates for the eye’s imperfections and even small muscle movements.

Adaptive optics won’t be in the doctor’s office tomorrow. The imaging machine is so large that it takes up an entire room and one component costs $150,000. But, says Carroll, five new machines are now being built that are smaller and cheaper.

Colorblindness primarily afflicts males because the genes for red and green cones are on the X chromosome, and males have only one copy of this chromosome. Females, on the other hand, have a second X chromosome that serves as a backup if something goes wrong with the first.

About one man in ten is colorblind.

Carroll, J. et al. Functional photoreceptor loss revealed with adaptive optics: An alternative cause for color blindness. Published online in Proceedings of the National Academy of Sciences (May 17, 2004).

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