Researchers have engineered mice to express an additional photoreceptor, a transformation that may mimic the evolution of trichromatic vision in primates, reports this week's Science. In the study, mice that express a human cone pigment sensitive to long-wavelength light can see colors that normal mice cannot."What this shows is that animals can develop quite sophisticated discrimination capabilities just by inserting a new class of receptors at the very front end of the visual system," said David Williams of the University of Rochester in New York, who was not involved in the study. "That's really fundamentally important in understanding how sensory systems develop."Mice normally have two types of cone photoreceptors -- blue- and green-sensitive -- which gives them dichromatic vision. Many primates have trichromatic vision arising from the addition of a third, red-sensitive photopigment. In Old World primates, this third pigment comes from a separate gene on the X chromosome....

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