Despite the formal character of the classical gene, it became the conviction of many geneticians in the 1920s, among them Morgans student Herman J. Muller, that genes had to be material particles but indirectly. The first direct evidence was obtained by Bridges (1916), who was able to show that a certain abnormal behavior of sex-linked genes, in other words genes that reside on the sex chromosomes, of D. melanogaster, namely, non-disjunction, corresponded to the analogous non-disjunction of the sex chromosomes. Further direct evidence was gained when Muller and Painter (1929) and Dobzhansky (1929) demonstrated that X-ray-induced structural changes of the chromosomes were associated with corresponding changes in the linkage relations of the genes. So the concept of gene was defined as is unit of heredity. But the experimental practice of X-raying alone could not open the path to a material characterization of genes as units of heredity. Meanwhile, cytological work had also added credence to the materiality of genes-on-chromosomes. At the same time, however, it further complicated the notion of the classical gene. They neoclassical period beganin the early 1940s, withwork in formal genetics showing that genes could be dissected into contiguous segments by genetic recombination. Hence, they were not dimensionless points but entities with length. These observations were made first in D. melanogaster (Oliver 1940) and then in microbial fungi. If genes had length, however, they must be long molecules of some sort, and the question was whether those molecules were proteins or DNA, the two major molecular constituents of the chromosomes. In 1941 the ‘one gene–one enzyme’ hypothesis, which helped to forge an experimental association between biochemistry and genetics, was born. George Beadle and Edward Tatum chose to attack the problem of gene action by genetic analysis of a known biochemical process. They produced and isolated mutant strains of the fungus Neurospora each unable to synthesize one of several chemicals involved in a single biosynthetic pathway. The first definition of the gene as a functional unit followed from the discovery that individual genes are responsible for the production of specific proteins.
èIs gene is material entities? Protein or DNA?
èWhat is the nature of genes and their mechanism of action?
