"Variety is the spice of life," or so the saying goes. In fact, it is probably more precise to say that variety is the key to life. It is genetic variation that contributes to the diversity in phenotype that provides for richness in human variation, and it is genetic variation that gives evolution the tool that it needs for selection and for trying out different combinations of alleles and genes.

Some variation is directly observable. Some examples of human genetic variants include the widow's-peak hairline, which is dominant to nonwidow's peak; free earlobe, which is dominant to attached earlobe; facial dimples, which are dominant to no facial dimples; and tongue-rolling, which is dominant to non-tongue-rolling. Another example is the ability to taste phenylthio-carbamide (PTC), a bitter-tasting substance. Seven out of ten people can taste the bitterness in PTC paper, and the ability to taste...