The Women With Superhuman Vision
16th September 2014
The question of whether we all see the same colours has a long history in philosophy and science. In the past, there seemed little reason to expect huge differences. We know that almost everyone has three types of “cone cells” in their retina that each respond to a different bandwidth of light. The colour of an object depends on the particular combination of those signals, but although the exact sensitivity may vary between people, overall one person’s colours should roughly match another person’s. The exceptions were thought to be colour-blind people, where one of the cones is faulty. With reduced sensitivity at certain wavelengths, they struggle to tell the difference between reds and greens, for instance.
In theory, though, it could go the other way: according to some estimates, an extra cone would offer a hundred different variants to each colour that humans normally see. We know that this happens in nature: zebrafinches and goldfish both have a fourth cone that seems to help them differentiate apparently identical colours. About 20 years ago Gabriele Jordan at the University of Newcastle and John Mollon at the University of Cambridge proposed a way that it might be possible in humans too.
The crux of Jordan’s argument lay in the fact that the gene for our red and green cone types lies on the X chromosome. Since women have two X chromosomes, they could potentially carry two different versions of the gene, each encoding for a cone that is sensitive to slightly different parts of the spectrum. In addition to the other two, unaffected cones, they would therefore have four in total – making them a “tetrachromat”. For these reasons, it’s thought to be a condition exclusive to women, though researchers can’t totally rule out the possibility that men may somehow inherit it too.