THE DEBATE OVER whether experimental gene therapy is too dangerous to be ethically applied to humans is only one among many knotty ethical controversies that surround genetic engineering. Genetic engineering refers to the alteration of genetic material—DNA, genes, and chromosomes—to change human traits, such as susceptibility to certain diseases and disorders. Gene therapy is one type of genetic engineering.

Human DNA, the raw material of life, is composed of billions of chemical molecules, called bases, linked together in particular sequences. DNA, in turn, makes up the bodies in the cell nucleus called chromosomes, and chromosomes are divided into separately functioning segments known as genes. The nucleii of trillions of cells in a person's body contain identical copies of that person's unique DNA, including anywhere from forty thousand to one hundred thousand genes. Genes contain chemical operating instructions for human functions...