96 Nature of the Genetic Material 



analysis, the ideas of a different type are based upon genetic facts, 

 though it seems to me that the morphology and the biochemistry of 

 the chromosomes are pointing in the same direction. 



A. THE THEORY OF THE GENE 



The theory of the gene goes back to the desire, visible in all 

 pre-Mendelian speculations of the epigenetic type, to explain heredity 

 by proper hereditary corpuscles in the cells which could be moved 

 around in the proper way to meet the needs of the respective theory. 

 With the discovery of mitosis these corpuscles were put in linear 

 order into the chromosomes (Roux, 1883), and Mendelian genetics 

 revived them as genes (Johannsen's term). The experimental under- 

 pinning of these speculations begins with Boveri-Sutton's proof that 

 Mendelian segregation and recombination is the consequence of the 

 location of Mendelizing factors within a pair of homologous chromo- 

 somes. It reaches its climax with the crossing-over studies of the 

 Morgan group, which proved the exact order, linear arrangement, 

 and localization of Mendelizing alleles in the chromosomes; and it 

 ends with the proof (Painter) that the visible arrangement of struc- 

 tures of the salivary gland chromosomes agrees exactly with the 

 results of genetic localization. In the minds of geneticists and non- 

 geneticists alike, this established the gene as the unit of heredity, 

 even the unit of life, being corpuscular, individual, independent, self- 

 duplicating, but capable of mutational change, beadlike, as in a 

 string of beads and imbedded in the chromosome. 



Though this was and is clearly the idea of a gene, its definition, 

 as derived from experiments by the Morgan group, was less concrete. 

 By definition the gene was the smallest section of a chromosome 

 within which no crossover break can occur. But this definition does 

 not make much sense, for it is based upon a criterion which cannot 

 be stated in absolute terms. The distance between two genes is 

 measured by the percentage incidence of a crossover break between 

 them. The smaller the distance the more improbable a break and 

 the larger the number of individuals needed to discover a break. 

 It is well known that a number of apparently unitary loci have been 

 separated by breaks when sufficiently large numbers were used and 

 the tendency for crossover breaks increased by proper experimenta- 

 tion. Where is the limit for this? Since this question cannot be 

 answered, the crossover definition of the gene is not possible. It is 

 important to realize this fact, because this very definition has fre- 



