Saturday, December 25, 2010

Life Lines 92 -- Color Blindness

The correct designation today is impaired color vision because almost all persons with this genetic defect see some color, especially blues and yellows. About 6 to 8 percent of American males are red-green color deficient. They confuse these colors with each other or with grays. I remember demonstrating this in one class when I drew with red on a slate blackboard and one of my teaching assistants who had red-green color deficiency could not see what I had written

Color deficiency was first considered a hereditary defect by the founder of atomic theory, John Dalton in 1794. He had his eyes preserved after his death and a few years ago they were analyzed, the genes removed and cloned, and the mutation that caused his deficiency was similar to the one that effects many hundreds of thousands of people of Celtic ancestry today. For reasons not known, Celtic people have much higher frequencies of color deficiency (up to 20 percent of the males in some Irish villages). It is inherited from one’s normal visioned mother. Half of her sons are at risk and the father is normal. That is because the gene is on the X chromosome. The mother is usually a carrier (heterozygous is the fancy term). The father supplies a Y chromosome to his son and not an X. The Y has very few genes on it and thus sons are vulnerable to what’s on the X they got from their mother.

There are four major forms of red-greed color deficiency. The most extreme are protanopes and deuteranopes. Those are your classic cases of the guys who need a friend or a relative to help pick out their clothes. The former messes up the reds and the latter messes up the greens. A milder form of each results in protanomaly and deuteranomaly. Both the protan and deutan genes have been sequenced for their DNA. These genes deposit pigments in the cone cells of the retina. The rest of the retinal cells, rod cells, give us our night vision of white, grays, and black.

I once checked out a family with a color blind father and a carrier mother and they had a colorblind daughter and a color blind son among their growing family. It’s easy to do with Ishihara plates which are numbers made of colored dots against a sea of colored dots. My favorite moment is giving a normal person an Ishihara chart that shows a number only red-green color deficient people see. The usual response: “You mean there really is a number there?”

One of my students, Shari Cohn, studied female carriers and showed that under reduced lighting conditions such females, tested one eye at a time, make mistakes more frequently than women who do not have color deficiency in the family. This is because all females are a mixture of cells, about half expressing their father’s X and the other half expressing their mother’s X. They are natural mosaics. The movement of their eyes and the splitting of signals from each eye into the two hemispheres of the brain permits such women carriers to see normal color. I called my son to see whether my grandson got the gene from his other grandfather (who was colorblind) through his mother. My grandson couldn’t read numbers then but he knew his colors.

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