The human genome is composed of roughly three billion DNA base pairs. Each somatic cell of the body contains two copies of the entire genome, one inherited from the mother through the egg, and the other inherited from the father through the sperm. The DNA encodes a library of biochemical messages called genes, perhaps 60,000 in all. All of the genetic messages, no doubt, are important to the health of the organism, and it is likely true that every disease state in the human involves some compromise or redistribution of the biochemical agents produced by the genes. In other words, genes are involved at some level in virtually every human disease. Even so, most diseases are not inherited.

The genes are packaged into chromosomes. Each somatic cell contains 46 chromosomes; two copies of each of the 22 autosomes (numbered 1 through 22), and a pair of sex chromosomes (X and Y...