Color blindness

July 6, 1999

--3 color receptors, for red, green and blue, a theory that goes back to Young and Helmholz in 1802. John Dalton, who developed the atomic theory of chemistry, was colorblind and did some work on this. He thought the aqueosu humor in his eye was colored blue. When he died, they checked it and it was normal color(clear). Later DNA was extracted from with eye, showing he as missing the green opsin gene. Dalton could distinguish 2 main colors in teh spectrum: ROYG was one and BIV the other.

--protan: red; deutan=green, tritan = blue. Can either be completely unable to see the color due to absense of teh pigment (e.g. protanopia) or see it anomalously due to altered absorbtion spectrum of the pigment (e.g protanomaly). Also: trichromat=able to see all 3 colors; dichromat: can see only 2 colors

--3 types of cones in retina, one for each color. All use the same pigment 11-cis-retinal, but different protein carriers, the opsins. Peak sensitivity is about 565, 530, 440 nm respectively for red, green, blue. Some variations based on method of assay.

--red/greed colorblindness (protan and deutan) known to be X-linked; tritan is autosomal. Red/green colorblindness gene was the first gene mapped to a mammalian chromosome in 1911 by Wilson.

--Nathans et al. (1986: Science 232:193-202 and 203-210) cloned and sequenced the opsin genes. Found on the X chromosome 1 copy of the red gene adjacent to 1-4 copies of the green gene. Color vision defects are related to mutations and hybrid genes here: which genes are missing determines protan (red) vs. deutan(green). Also, many anomalous color vision people have a hybrid gene: red/green or green/red. The exact position of the splice affects the type of color defect.

--there are polymorphisms for red and green known that don't seem to affect color vision too much. There are also phenotypic polymorphisms based on "greenpoint" and "bluepoint", the points in the spectrum that seem to be pure green and pure blue.

--color vision is often assessed by a Nagel anomaloscope, an instrument that shows pure yellow light (591 nm) of variable intensity on one side, and a experimenter-varied ratio of red (644 nm) and green (541 nm) on the other. The experimenter sets the red/green ratio, and the subject varies the yellow intensity until it seems to match the supplied red/green mix. Normal vision people can match the yellow with only a very narrow range of red/green ratios; dichromats can match the yellow at almost any red/green ratio, but variation occurs with the intensity of yellow light that matches. A person missing the red pigment needs a lot of red to match, etc. Anolamies (reduced red or green) can match over a greater range than normals, but not the entire range.

--Deuteroanomaly (reduced ability to see green is about 50% of people, with deuteropia (no green), protoanomlay (reduced red vision), and protopia (no red vision) split the remaining 50% equally. About 8% of American whites have detectable color vision problems. Interestingly, about 15% have altered color genes--many of these can see color normally.

--there is a polymorphism in the red gene common in humans (but not apparently found in apes) either Ser or Ala at position 180. These give maximal absorbsions of 557 or 552 nm. These people respond differently on the anomaloscope, and thus apparently see color somewaht differently.

--the blue gene is much less similar to the red and green genes: red and green opsins are 96% homologous (15 amino acid differences out of 162 total), while blue is 43% with red or green. The few know defects are point mutations with dominant effect, not simple inactivations or deletions. Blue/yellow colorblindness (tritanopia) is much rarer than red/green.

--the Old World monkeys and great apes share the same color receptors that we have; New World monkeys have 1 gene for red and green. But squirrel monkeys can have up to 6 different color genes--varies with the individual.

--Current theory is that only the first 2 genes in the red/green locus are expressed, usually the red and the first green. But sometimes the red or the green has a hybrid in its place, which can lead to colorblindness. However, a hyrid gene in teh third position seems to have no effect (very recent theory however).

--another defect, achromatopsia, is complete absence of color vision; unable to distinguish any colors.