The modern science of genetics can be traced to the research of a Moravian monk, Gregor Mendel, in the mid-1800s. Mendel was able to develop a series of laws that described mathematically the way hereditary characteristics are passed along from one generation to the next. These laws could best be understood by assuming that hereditary characteristics are contained in discrete "packages" in an organism. These "packages" were called by a variety of names such as unit character, gemmule, biophere, pangene, and eventually, gene.

By whatever name these "packages" were called, however, the term had no concrete referent. It was simply used to refer to some abstract concept that was needed in order to understand the laws of heredity.

The story of genetics during the twentieth century is, in one sense, an effort to discover more specifically what a gene is. An important breakthrough...