Kenzi And Roni S Paper

Roni Miller and Kenzi Mendlik
Developmental genetics paper
3/10/09

Color blindness

A human eye can see many different colors. There are two types of light receptor cells in the eye, rods and cones. Rods help people see in dim light while cones are in charge of vision in bright light. There are three different types of cones and the combination of these three cones are responsible for normal color vision. The human brain sees the world through three primary colors; red, green and blue. Color blindness is caused by the lack of a normal gene copy or an abnormal gene of the colored-vision pigment genes. Researchers predicted that all the pigmented genes in the eye would have similarities because they have a common evolutionary origin. They found that one single-stranded DNA will bind to a similar, complementary strand that will isolate a gene that will be used to search for related DNA sequences (1).

A research was done to identify the color blindness genes and evolution. Researchers used a visual pigment gene, rhodopsin, to find a human gene that is used to see dim light but not color. From this gene, they were able to discover three similar DNA sequences. On the X chromosome, there were the red and green pigment genes while the blue pigment gene was located on the number 7 chromosome. From looking at these genes, the common DNA segment of these produced three genes; rhodopsin gene, blue pigment gene, and one that duplicated to form the green and red pigment genes (1).

Researchers also found from the experiment mentioned above, that people with normal color vision have anywhere from two to three copies of the green pigment gene on the X chromosome. When these green pigment genes have uneven exchanges of DNA between the paired chromosomes, it causes the chromosomes to lack a color vision gene. The exchange of DNA can cause a mixture between the red and green pigment genes to occur which could lead to color blindness. The mixture of the red and green pigments cause people to produce pigments with different light absorption characteristics than those who have just one normal pigment (1).

Color blindness occurs more in men than in women. It affects about eight to ten percent of men. The different forms of color blindness are complete absence of color vision, red-green, and blue-yellow color vision defects. The most common form of color blindness is the red-green defect. If a person has a red-green color defect, they have difficulty seeing different tints of red and green, blue-yellow defects cause difficulty in seeing different tints of blue and green. The absence of color vision is rare; it causes individuals to only see images in black, white, or gray (3).

The red pigments are long wavelength sensitive (LWS), green pigments are middle wavelength sensitive (MWS), and blue pigments are short wavelength sensitive (SWS). All pigments are part of a specific evolutionary group, LWS/MWS group. If an individual only has SWS pigments and either LWS or MWS, then that individual will have red-green color blindness. Each color pigment has a chromophore, a molecule that produces color through selective absorption of light, and a transmembrane protein, opsin. Opsin is programmed by a specific opsin gene. The opsin genes of the LWS, MWS, and SWS are determined by amino acid sequences. It was found that there are five critical sites on the amino acid sequence for the LWS, MWS, and SWS. Experiments were done that tested the amino acid sequences of these five critical sites to see if it caused interference in the red-green color vision. Experiments on wavelengths also showed that LWS pigments evolved from MWS pigments (2).

Red-green color defect is the most common form of color blindness, but is more likely among men than women. People affected with this have trouble distinguishing between red and green shades, and view these colors differently than a person with normal eye sight. This defect is inherited in an X-linked recessive manner during infantry, and the genes responsible are OPN1LW, encoding the red pigment and OPN1MW, encoding the green pigment. Since the females have chromosomal make-up of XX, the mother is the carrier of the mutation of one of these genes which she can then pass on to her children. In fact, the chance of the mother transmitting the mutation to her child in each pregnancy is 50%. The reason males have a greater risk of inheriting one of these mutated genes is because males only have one X chromosome and females have two X chromosomes. Therefore, males will be red-green color blind if their only X chromosome is defective, while females will be red-green color blind only if both of their X chromosomes are defective (3).

Blue-yellow color defects are rarer than red-green vision defects. The people affected with this defect have problems distinguishing between blue and green shades, and would see these colors differently if they were not to have this defect. The gene coding for this defect is on chromosome 7, which is distributed equally among men and women. So, this defect is caused by a simple mutation in this gene on chromosome 7 (3).

Both of these two vision defects only have an effect on a person’s color awareness, but they do not affect the sharpness of the person’s vision.
Experiments were done on Tritan color vision deficiency, vision disorder with mutations in the SWS or S-cone pigment gene. This disorder has different degrees of color blindness but all the individuals have the same underlying mutation. Tritan color vision deficiency is caused by mutations in the S-cone opsin gene. The S-cone opsin gene encodes for the protein component of the S-cone pigment. Mutations at this specific opsin gene cause individuals to have four different amino acid substitutions which happens at the amino acid site that is arranged in one of the transmembrane alpha helices. This can lead to a change in the folding, processing, or stability of the encoded opsin. A mutation in which the amino acid substitution occurred in rhodopsin, the rod pigment, caused retinal degeneration (4).

Sources:
1. Miller, J.A. “The Genes behind Vision’s Palette.” Science News. April 19, 1986, Vol 129, No.16, p.246.
2. Yokoyama, Shozo and Bernhard, F. Radlwimmer “Molecular Genetics and Evolution of Red Green Color Vision in Vertebrates.” Genetics. August 2001,Vol. 158.
3. Botstein, David. “The Molecular Biology of Color Vision.” Science. April 11, 1986, Vol. 232, No. 4747, p. 142-143.
4. Barass, Rigmor, C., Joseph Carroll, Karen L. Gunther, Mina Ching, David R. Williams, David H. Foster, and Maureen Neitz. “Adaptive optics retinal imaging reveals S-cone dystrophy in tritan color-vision deficiency.” J Opt Soc Am A Opt Image Sci Vis. May 2007, Vol. 24, No. 5.

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