Men are more likely to fail to distinguish this thing

There are about 10 million American men with leukemia. Seven percent of the total leukemia population cannot tell red from green, or see red and green differently than most people do. This is the most common form of color blindness, but it affects only 4 percent of women. The greater prevalence of color blindness in men suggests that it, like hemophilia, is carried by the X chromosome, of which men have only one. (As in hemophilia, women are protected because they have two X chromosomes; the normal gene on one chromosome can compensate for the defective gene on the other.)

Wald and his colleagues found that in color-blind men, the red or green cone cells function abnormally or not at all. Wald believes that the genes encoding red and green receptors in these men have changed, and these genes are located adjacent to each other on the X chromosome. This tandem arrangement, Nathans has confirmed, can be attributed to the duplication of primate DNA segments 40 million years ago.

Primates in South America, which split from Africa around that time, have only one functional copy of the red-green gene, much like color-blind men. But in primates in Europe (Europe, Asia, and Africa)—the ancestors of African monkeys and apes and humans—an original red-green gene may have been duplicated and slightly diverged in sequence, resulting in separate receptors for red and green. Starting with this scenario, Nathans found that the DNA sequences of the genes for red and green receptors differ by only 2%. This is evidence that they had a common origin and then diverged.

Nathans himself is not color blind. Before using his own DNA, he had his color vision thoroughly checked to make sure it was normal. But one of his initial findings caused trouble: after doing an end-to-end sequence of his X chromosomes, he found not only two copies of the genes for red and green receptors, but also an extra copy of the gene for green receptors. He realized this was the explanation for the prevalence of color blindness. Because the DNA sequences of the red and green receptor genes are so similar, and because they are arranged end-to-end, it is easy for errors to occur when the egg and sperm are produced, that is, the duplication and exchange of genetic material between chromosomes.

An X chromosome - like Nathans's own - might receive an extra green receptor gene, or even two. This would do no harm. But then the other chromosome with which it swapped bits of genetic information would be left with only one red receptor gene. A male who inherited this slightly shortened chromosome would be color blind, deprived of the genetic information needed to produce green receptors.

Over 95% of the variation in human color vision involves the participation of the red and green receptors in the eye. It is very rare for anyone - male or female - to be "color blind" to the blue portion of the spectrum. Nathans proposed a genetic explanation for this phenomenon. He showed that the gene code for the blue receptor is located on chromosome 7, shared equally by males and females, and that this gene has no neighbors with very similar DNA sequences. Tritan color blindness is caused by a simple mutation in this gene.