Color Perception And Blindness: The Genetic Basis

I’ve written a number of times before about the Bayer pattern common to most image sensors (notably save those from Foveon), specifically the eye’s cones’ green-spectrum percentage bias that are the Bayer filter allocation underpinnings. In (unsuccessfully) striving to work my way through my periodical reading pile over the weekend, I dove into the April 2009 issue of Scientific American and discovered an excellent article on the genetic foundation for this phenomenon.

"Color Vision: How Our Eyes Reflect Primate Evolution" discusses, among other things, the trichromatic pigment pattern that’s fairly unique to Old World primates (including, at least according to my particular belief system, human beings). In contrast, only a subset of females in New World primates (from Central and South America) are trichromatic from a cone pigment distribution standpoint; the remainder are dichromatic. However, this seeming disadvantage doesn’t necessarily mean that dichromatic- pigmented primates are substantially color perception-disadvantaged versus their trichromatic relatives. The brain, after all, receives and processes the cones’ provided data, and as the article states, "certain retinal and brain mechanisms involved in longer-wavelength color vision may be highly plastic."

The article also describes in detail (and in a timely nod to the fact that this year marks the 150^th anniversary of the publication of Charles Darwin’s On The Origin Of Species) how trichromacy initially probably appeared as a genetic mutation that was subsequently reinforced by means of natural selection. As the authors write:

In the case of primate color vision, trichromacy based on the "new" M [editor note: medium wavelength] and L [long wavelength] pigments (along with the S [short wavelength] pigment) presumably conferred a selective advantage over dichromats in some environments. The colors of ripe fruit, for example, frequently contrast with the surrounding foliage, but dichromats are less able to see such contrast because they have low sensitivity to color differences in the red, yellow and green regions of the visual spectrum. An improved ability to identify edible fruit would likely aid the survival of individuals harboring the mutations that confer trichromacy and lead to the spread of those mutant genes in the population.

Here’s some more background on the above-mentioned wavelengths:

The short-wavelength (S) pigment absorbs light maximally at wavelengths of about 430 nanometers (a nanometer is one billionth of a meter), the medium-wavelength (M) pigment maximally absorbs light at approximately 530 nanometers, and the long-wavelength (L) pigment absorbs light maximally at 560 nanometers. (For context, wavelengths of 470, 520 and 580 nanometers correspond to hues that the typical human perceives as blue, green and yellow, respectively.)

And finally, the article provides the genetic basis for a phenomenon I mentioned a few months ago, i.e. the higher probability that men (versus women) will be colorblind:

Experiments also showed that the M- and L-pigment genes sit next to each other on the X chromosome, one of the two sex chromosomes. (Men have one X and one Y, whereas women have two Xs.) This location came as no surprise, because a common anomaly in human color perception, red-green color blindness, had long been known to occur more often in men than in women and to be inherited in a pattern indicating that the responsible genes reside on the X chromosome. The S-pigment gene, in contrast, is located on chromosome 7, and its sequence shows that the encoded S pigment is related only distantly to the M and L pigments.

A great read (and for that matter, a great magazine); highly recommended.