The rise of molecular biology and the modern concept of the gene provides an especially clear illustration of the progression of scientific understanding in the life sciences. The earliest model of the gene was derived from Mendel’s pea plant experiments in the 1860s. Mendel concluded that these plants exhibited dominant and recessive traits that were inherited. The key concept at this stage was the trait itself, with no attempt to conceptualize the physical mechanism by which the trait was passed on from generation to generation (Derry, 1999). By the time Mendel’s work became known to the scientific world, cell biologists with newly improved microscopes had identified the threadlike structures in the nuclei of cells called chromosomes, which soon became known through experiments as the carriers of hereditary information. It was quickly recognized that some traits, eventually to be called genes, were inherited together (linked), and that the linkage was due to those genes being located on the same chromosome. Using breeding experiments with strains of various organisms, some having altered (mutated) genes, geneticists began to map various genes to their chromosomes. But there was still no conceptualization of the nature or structure of the genes themselves.

The next refinement of the model was to identify the gene as a molecular structure, which required the development of biochemical and physical techniques for working with large, complex molecules. Although other experiments at nearly the same time pointed to deoxyribonucleic acid (DNA) as carrying genetic information, the structure of DNA was not yet known. Scientists of the day were reluctant to accept the conclusion that DNA is the primary hereditary material because a molecule composed of only four base units, it was thought, could hardly store all the information about an organism’s features. Moreover, there was no mechanism known for passing such information on from one generation to the next.

It was these developments that led to the watershed discovery by Watson and Crick (1953) (and related work of a host of other scientists in the emerging field of molecular biology) of the DNA double helix and the subsequent evidence that genes are lengths of DNA composed of specific sequences of its four basic elements. The double helix structure that Watson and Crick discovered from analyzing DNA X-ray diffraction data also was