Monkeys (and people) can't tell red from green when their L-opsin gene isn't working well. So an obvious solution is to simply add a working copy of the gene to their eyes. Of course, it sounds simple, but it took a little ingenuity and a little luck to pull off.
The first hard part was communicating with the monkeys. Unlike King Louie of the Jungle Book, real monkeys don't speak human. They can't say what they see
However, they like juice. So, scientists had already designed a test based on that. They took a board with multiple gray dots and displayed colors in one of three areas.
If the monkey touched the correct patch with color, they got some juice. However, monkeys with a broken L-opsin gene remained thirsty when red and green were shown. They couldn't distinguish between the two colors, and that confirmed that they were colorblind.
Viruses can inject genes
The next step was getting a working copy of L-opsin into these colorblind monkeys. That meant getting genes into the cells of the monkey's eyes.
To do this, scientists used two common tools: a syringe and a virus. The syringe was used by a surgeon to inject the virus into the eyes of anesthetized monkeys. The virus was used to inject the L-opsin gene into the cells of the monkey's eye.
While the syringe is familiar to anybody who has received a shot, the use of a virus may not make much sense. How and why did scientists use a virus to cure colorblindness?
The why mostly has to do with getting past the protective membrane that surrounds a cell. The membrane is kind of like the skin of the cell and keeps unwanted stuff out. But scientists wanted to get the L-opsin gene in. So they used something like a needle that has evolved to do just this -- a virus.
Scientists have used viruses for getting genes into cells for quite awhile, so the technique wasn't new. Normally, these viruses work by attaching to cells and injecting their own genes.
But scientists didn't want to inject virus genes; that would probably make things worse. So, they substituted some of the virus's genes with what they wanted: the working L-opsin gene.
To recap, surgeons injected viruses into the eyes. The viruses then injected the genes into the cells.
Amazingly, the monkeys with the new gene passed the color test. They got a taste of juice when red or green appeared -- a pretty and sweet result. This told scientists that the monkeys were cured of their colorblindness.
Scientists used some not-so-new tools to address a new problem. But science isn't just about being knowledgeable or clever. It often requires a little bit of luck, too.
Most scientists did not think that curing adult monkeys of colorblindness would be possible. In the 1960s, scientists saw that eyes needed to be stimulated early in life to develop properly. This led to the belief that things have to go right at an early age for an eye to work properly.
Most scientists would have thought that it was impossible to add a gene to an adult monkey to cure a genetic defect related to sight. It just goes to show that sometimes believing leads to seeing.