Inhibiting the development of polyps and cancers, finding and removing premalignant polyps, and testing individuals at high risk may reduce colon cancer-related morbidity and mortality. Colon cancer occurs less commonly in individuals whose diets are high in calcium and folate, who take multivitamins, and who maintain high-fiber and low-fat diets.

Non-steroidal anti-inflammatory medications like aspirin may reduce the numbers of polyps, particularly in families with FAP. Colonoscopy can identify polyps that may be premalignant and can facilitate polyp removal. It is recommended that all individuals have a colonoscopy at age fifty. High-risk patients, such as those with inflammatory bowel disease, FAP, or HNPCC, should have screening initiated at an earlier age and repeat exams at shorter time intervals. see also Apoptosis; Breast Cancer; Cancer; Carcinogens; Cell Cycle; DNA Repair; Genetic Testing; Mutation; Oncogenes; Tumor Suppressor Genes.

David E. Loren and Michael L. Kochman i Bibliography

Giardiello, Francis M., Jill D. Brensinger, and Gloria M. Petersen. "AGA Technical Review on Hereditary Colorectal Cancer and Genetic Testing." Gastroenterology 121 (2001): 198-213.

Yamada, Tadataka, et al., eds. Textbook of Gastroenterology, 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins, 1999.

Internet Resource

"The Burden of Gastrointestinal Diseases." American Gastrointestinal Association. May 2001. <>.

Color Vision one billionth of a meter

Sight is a complex process that results when visible light, a narrow band of nanometer 10~9 meters; the electromagnetic spectrum between 400 and 700 nanometers (nm), is converted into signals that can be interpreted by the brain. This process involves special light-sensitive cells called photoreceptors that are located in the retina, a thin structure that lines the inside of the eye. These cells capture packets of light, called photons, and transform their energy into signals that are transported from the eye to the occipital cortex, the portion of the brain that allows us to interpret these signals as sight.

Normal human color vision is trichromatic (based on the perception of three primary colors) and requires three types of photoreceptor cells, called cones, each of which contains a different photopigment. Each photopigment



Incidence in

Male Population

Color Vision



I. Hereditary


1. Normal


2. Deuteranomalous



3. Protanomalous



4. Tritanomalous




1. Deuteranopes



2. Protanopes



3. Tritanopes



Monochromats (achromats)

1. Typical (rod monochromats)



2. Atypical (cone monochromats)



II. Acquired

1. Tritan (blue-yellow)

2. Protan-deutan (red-green)

Table 1. Adapted from American Academy of Ophthalmology, 1995.

Table 1. Adapted from American Academy of Ophthalmology, 1995.

absorbs particular wavelengths of light in the short (blue, 440-nm), middle (green, 545-nm), or long (red, 560-nm) wavelength region of the visible spectrum. About 7 percent of all cones are blue-sensitive, 37 percent are green-sensitive, and 56 percent are red-sensitive. These cones are the basic mediators of color vision. If one or more of their pigments is missing, color blindness results. Rod cells, unlike cones, detect light intensity but not color. The photopigment in rod cells is called rhodopsin.

The spectral sensitivity of the cone photopigments is intimately related to the structure of the pigment molecules. These are concentrated in the photoreceptor outer segment, the portion of the cell containing the photo-transduction machinery. Each pigment molecule consists of an opsin protein and a chromophore (11-cis-retinal), which is a derivative of vitamin A. Photon absorption by the pigment molecules causes a change in the shape of the chromophore, which initiates the processes that lead to vision.

The different opsins of the cone photopigments and of the rod photopigment are encoded by four separate genes, the BCP (blue cone pigment), GCP (green cone pigment), RCP (red cone pigment), and RHO (rhodopsin) genes. The genes encoding the blue cone and rod pigments reside on the long arms (called the q arms) of chromosome 7 and chromosome 3, respectively. The genes encoding the red and green cone pigments reside on the q arm of the X chromosome.

Color vision defects may be divided into two groups, hereditary and acquired. Hereditary color vision defects are almost always "red-green" and affect 8 percent of males and 0.5 percent of females. Acquired defects are more often "blue-yellow," and affect males and females equally. Hereditary defects are typically bilateral (affecting both eyes), while acquired defects may affect one eye only and are often asymmetric. Hereditary color vision defects tend to remain stable throughout life and are usually not associated with other retinal or optic nerve pathology. Acquired defects, however, may pathology disease have a more variable course and are frequently associated with observable process

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