Differences in iris color have been attributed to such causes as the temperature of the brain and eyes. Some people have stressed differences between dark-eyed and light-eyed populations and have suggested that eye color is related to general traits such as temperament or intellect. But, toward the middle of the nineteenth century, it had become clear that iris color was due to iris pigment, that this pigment developed soon after birth, and that the final quantity and distribution of the pigment was a hereditary trait.
Originally, iris color was thought to be a simple trait—one governed by a single gene with multiple forms, or alleles, corresponding to each color. In this scheme, blue was thought to be recessive, requiring two copies of
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