Some 10 million American menfully 7 percent of the male populationeither cannot distinguish red from green, or see red and green differently from most people. This is the commonest form of color blindness, but it affects only .4 percent of women. The fact that color blindness is so much more prevalent among men implies that, like hemophilia, it is carried on the X chromosome, of which men have only one copy. (As in hemophilia, women are protected because they have two X chromosomes; a normal gene on one chromosome can often make up for a defective gene on the other.)
Wald and others had found that in color-blind men, the green or red cones worked improperly or not at all. Wald suggested that the genes for the red and green receptors were altered in these men. He also thought that these genes must lie near each other on the X chromosome. This tandem arrangementwhich Nathans confirmedprobably results from the duplication of a DNA fragment in primates that occurred some 40 million years ago.
The primates of South America, which broke from the continent of Africa at about that time, possess only a single functional copy of a red or green gene, much like color-blind men. But in Old World primatesthe monkeys and apes of Africa and the ancestors of humansa primordial red-green gene must have duplicated and then diverged slightly in sequence, leading to separate receptors of the red and green type.
In keeping with this picture, Nathans found that the DNA sequences of the genes for red and green receptors differ by only 2 percentevidence of their common origin and recent divergence.
Nathans himself is not color-blind. Before using his own DNA, he thoroughly tested his color vision to ensure that it was normal. Nevertheless, one of his initial findings presented a puzzle: Lying head to tail along his X chromosome were not just the two genes for the red and green receptors, but also an extra copy of the green receptor gene.
Here was the explanation for the prevalence of color blindness, he realized. Because the DNA sequences of the red and green receptor genes are so similar, and because they lie head to tail, it is easy for mistakes to occur during the development of egg and sperm, as genetic material is replicated and exchanged between chromosomes.
One X chromosomelike Nathans'may receive an extra green receptor gene, for instance, or maybe even two. This does no harm. But then the other chromosome with which it is exchanging bits of genetic information is left with only a red receptor gene. The man who inherits this slightly truncated chromosome will be color-blind, bereft of the genetic information needed to make a green receptor.
More than 95 percent of all variations in human color vision involve the red and green receptors in men's eyes. It is very rare for anyonemale or femaleto be "blind" to the blue end of the spectrum. Nathans provided a genetic explanation for this phenomenon. He showed that the gene coding for the blue receptor lies on chromosome 7, which is shared equally by men and women, and that this gene does not have any neighbor whose DNA sequence is similar. Blue color blindness is caused by a simple mutation in this gene.
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