The X in Sex: How the X Chromosome Controls Our Lives by David Bainbridge

Author:David Bainbridge [Bainbridge, David]
Language: eng
Format: epub, pdf
Published: 2011-01-29T05:00:00+00:00

red, green, green, green

Whereas conversely these two arrays would lead to complete red-green color blindness:

red, red, green, green

green, green, green, green

The strange spliced-together red-green hybrids can cause rather more vague forms of color blindness. For example, the effects of the array red, hybrid, green, green, green rather depend on the exact rung code of the hybrid gene. If it is more like a green gene than a red gene, then the man may be able to discern colors almost normally. If, however, the hybrid is more like a red gene, then he may be almost as color blind as if he did not have that gene at all.

This is why red-green color blindness can be either total or partial, depending on the first two genes on that X chromosome list. Actually, it is thought that there may be even more color-blind people than we realize, as many cases of partial color blindness may be so mild that they are never diagnosed. Certainly there seem to be more abnormal sets of cone-opsin genes in the male population than there are men who think they are color blind. And all this confusion has resulted because dividing human cells cannot cope with the almost identical red-and green-cone-opsin genes sitting right next to each other on the X chromosome. Like two colors plucked from the palette and hastily applied to the adjacent parts of the canvas, the red and green have a habit of smudging into each other.

Before we find out why red-green color blindness has been built into the human race, it is worth emphasizing just how common it is compared to other forms of color blindness. Like any part of the body, just about anything that can go wrong with color vision does go wrong, resulting in many different forms of color blindness. Yet compared to red-green color blindness all these other conditions are extremely rare. We have already seen that blue color blindness is very uncommon, and the same is true of most of the other possibilities. Sometimes, there is a complete failure of the red-green region of the X chromosome, so that people (usually men) are left with just rods and blue cones—a bit like nocturnal prosimian primates. Other people have no functional cones at all, and so they are extremely sensitive to bright light because all they have to see with are rods, and they are designed for dim light. This no-cone disorder is called “achromatopsia” and it is vanishingly rare in the general population. However, the condition is prevalent on a small island in the Pacific called Pingelap. It is thought that a violent tropical storm laid the island waste in the eighteenth century, leaving only one male survivor, and the island was subsequently repopulated with the progeny of this one man. Unfortunately, he carried a damaged version of a gene normally used to make a protein vital to the functioning of cone cells, so modern Pingelap is full of completely color-blind people. If we want to know

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