The discoveries of twentieth-century genetics in general, and the Human Genome Project in particular, have launched medicine on a whole new course. Rather than waiting for diseases to develop and then treating them with drugs or surgery, doctors are now embarked on finding the genetic causes of disease in the hope of fixing the malfunctioning gene before the illness even begins to show its early symptoms. In addition to repairing faulty genes, medical researchers now have within their grasp the ability to analyze individual genomes-the total genetic makeup of specific organisms-to see if they have a full complement of genes and to add healthy versions of those that are missing.

Mutation

Not all ailments are genetic diseases, but it is becoming increasingly apparent that genes play some role in almost everything that can go wrong with a human being. Defects in genes, or genes...