Notes on Colour

colour vision deficiency

Colour vision deficiency

People who have difficulty distinguishing the full range of hues are said to have colour vision deficiency. The most common forms of colour vision deficiency are deuteranopia and protanopia. These are often lumped together under the imprecise label red-green colour blindness.

The production of green-sensitive cones is controlled by a gene on the X chromosome. The production of the red-sensitive cones is controlled by another, nearby gene on the X chromosome. These are sex-linked genes because they occur on part of the X chromosome that has no match on the Y chromosome. Women have two X chromosomes; men have a Y chromosome and only one X chromosome. Consequently men are more likely than women to miss out on forms of these genes that produce normally-functioning cones. (I don’t know anything about the gene for blue-sensitive cones.)

If you don’t have a gene for normally-functioning green-sensitive cones, you may not be able to distinguish green hues—you may have deuteranopia. If you don’t have a gene for normally-functioning red-sensitive cones, you may not be able to distinguish red hues—you may have protanopia. If you don’t have a gene for normally-functioning blue-sensitive cones, you may not be able to distinguish blue hues—you may have tritanopia.

Simulation

It’s impossible to show somebody who does not have a colour vision deficiency what the world looks like to people who do—I’m not even sure what it would mean! Nevertheless, I have prepared a crude simulation to give you a very rough idea of how we might imagine it. The five buttons next to the photograph are supposed to represent normal vision, deuteranopia, protanopia, tritanopia and also achromatopsia, a complete inability to distinguish colours.

Canvas for rgb912.pde