People with red-green colour blindness find it difficult to tell red hues from green ones because of a fault in a single gene. Their inheritance robs them of one of the three types of colour-sensitive cone cells that give us colour vision. With modern technology, scientists might be able to insert a working copy of the gene into the eye of a colour-blind person, restoring full colour vision.
You might think that the brain and eye would need substantial rewiring to make use of the new hardware, but Katherine Mancuso from the University of Washington thinks otherwise. She has used gene therapy to give full colour vision to adult squirrel monkeys that had been red-green colour-blind since birth, opening up a world of formerly invisible reds and oranges, right in front of their eyes.
Her success proved that adding a third cone cell into a retina with just two isn’t as much of a technical challenge as it initially seems – the new cells slot in with all the ease of plug-and-play hardware. The experiments suggest that early mammals could have evolved three-colour vision simply by developing a third type of cone cell, with little in the way of extra genetic control or neural wiring. If anything, the third cone probably exploited the circuitry that was already in place to process the signals from one of its siblings.
The human eye has three types of cone cells – the S cones that are sensitive to short violet-ish wavelengths, M-cones that are sensitive to medium greenish-blue wavelengths, and L-cones that are sensitive to longer reddish wavelengths. Each type of cone cell contains a different light-sensitive pigment – an opsin – and each of these is produced by a single gene. Red-green colour-blindness is what happens when the genes for the M-opsin or the L-opsin are flawed.
In squirrel monkeys, females see a more colourful world than males. While they have the same three opsins that humans do, males lack the gene for L-opsin and can’t see red. Mancuso changed that by loading the human L-opsin gene into a virus and injecting it into the monkeys’ retinas. As a result, around 15-30% of each animal’s M-cones were also producing L-opsins. It’s a trick that other researchers have used to give mice the ability to see red and it clearly worked for monkeys too.