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Color vision is the ability of the eye to detect color in the environment. Specific colors are perceived by photoreceptors, or cones, located on the retina at the back of the eye; the type of cone determines the specific colors perceived. Humans and Old World primates have three types of cones, while most other mammals have only two. Birds have four types, which allow them to see ultraviolet light, and insects have the same number of cone types as humans, though not of the same type. The absence or malfunction of particular cones may result in color blindness.
Humans, Old World primates, and insects have trichromatic color vision. The human and primate retina contains three types of color receptors: red, blue, and green. Mixing these three colors together can produce all the colors trichromates can perceive. Insects also have three types of cones: green, blue and, instead of red, an ultraviolet light-sensitive one.
The color vision of dogs and most other mammals is dichromatic. Having only two types of cones in the eye—green and blue—means that the perception of color is poor compared to human, bird, and insect vision. Many mammals typically have superior senses other than vision. Dogs may not be able to see tiny spots of red blood distinctly, but they can generally smell even a minuscule amount of scent impossible for human olfactory senses to detect.
Birds, turtles, and fish have tetrachromatic color vision. They have four different types of cones in their eyes which perceive the blue, green, red, and ultraviolet light reflecting from objects. Many birds have plumage markings in the ultraviolet range that play a role in mating selection. Studies suggest that a small percentage of women may also have tetrachromatic color vision, though not to ultraviolet wavelengths. Two to 3 percent of women may have an extra photoreceptor located between red and green.
Color blindness occurs when a person is missing certain pigments in the retinas' cones. Approximately 8 percent of men and 4 percent of women have faulty color vision due to an inherited trait linked to the X chromosome. People with color-blindness often have difficulty distinguishing between red and green; this can cause problems with determining the colors in a traffic light or how "done" cooked meat is. The test generally used to determine color blindness involves asking the person to detect a number from a circle filled with dots of varying colors. Children too young to recognize numbers may be asked to point out shapes, such as stars, circles, or squares.