Some of the greatest controversies about genetic engineering involve the alteration of human genes. Even some people who see nothing wrong with changing the genes of plants or animals draw the line at altering the human species. They fear that such changes could eventually transform the very definition of humanity—and not necessarily for the better. Supporters of human gene alteration, however, say that it could eradicate presently incurable diseases and might, in time, provide humans with powers presently undreamed of outside of video games and comic books.

Scientists are already changing human genes on a limited basis as a way of treating disease. The first gene therapy took place in September 1990, and several hundred forms of gene therapy are currently undergoing human tests. Supporters say that gene therapy eventually offers as much promise of revolutionizing medicine as the discovery of...