Cones are less sensitive than rods to light, but the cones provide color vision and greater visual acuity, as described in the next section. During the day, therefore, the high light intensity bleaches out the rods, and color vision with high acuity is provided by the cones. Humans and other primates have trichromatic color vision (are trichromats). This means that our perception of a multitude of colors is produced by stimulation of only three types of cones. This fact is exploited by television screens and computer monitors, which display only red, green, and blue pixels. Interestingly, other mammals that are able to see colors get by with only two types of cones (they are dichromats).
The three different cones responsible for human color vision are designated blue, green, and red, according to the region of the visible spectrum in which each cone pigment absorbs light best (fig. 10.40). This is the cone's absorption maximum, and corresponds to wavelengths of 420 nanometers (nm) for the blue cones (also called short wavelengths, or S cones), 530 nm for the green cones (also called medium wavelength, or M cones), and 562 nm for red cones (also called long wavelengths, or L cones). The gene for the S cones is located on chromosome number 7, whereas the genes for the M and L cones are located on the long arm of the X chromosomes.
Each type of cone contains retinene, as in rhodopsin, but the retinene in the cones is associated with proteins called pho-topsins, which are different from the opsin in rods. It is the three different photopsin proteins (coded by three different genes) that gives each type of cone its unique absorption maximum.
■ Figure 10.40 The three types of cones. Each type contains retinene, but the protein with which the retinene is combined is different in each case. Thus, each different pigment absorbs light maximally at a different wavelength. Color vision is produced by the activity of these blue cones, green cones, and red cones.
PS Color blindness is caused by a congenital lack of one or more types of cones, usually the absence of either ^ the L (red) or M (green) cones. Since such people have only two functioning types of cones, they are dichro-mats. The absence of functioning M cones, a condition called deuteranopic, is the most common form of color blindness. The absence of L cones (protanopia) is less common, and the absence of S cones (tri-tanopia) is the least common. People who have only one cone in the middle to long wavelength region (M or L) have difficulty distinguishing reds from greens. Since the M and L cone pigments (photopsins) are coded on the X chromosome, and since men have only one X chromosome (and therefore cannot carry the trait in a recessive state—see chapter 20), such red-green color blindness is far more common in men (with an incidence of 8%) than in women (0.5%).
Suppose a person has become dark adapted in a photographic darkroom over a period of 20 minutes or longer, but needs light to examine some prints. Since rods do not absorb red light but red cones do, a red light in a photographic darkroom allows vision (because of the red cones) but does not cause bleaching of the rods. When the light is turned off, therefore, the rods will still be dark adapted and the person will still be able to see.
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