100 Nature of the Genetic Material 



is canceled by the presence of a chromosomal segment containing the 

 normal locus. Further, a mutant locus contained in a chromosome 

 fragment acts, as far as known, in the same way as if it were included 

 in a normal chromosome. Finally, in the majority of cases in which 

 a homozygous recessive contains an additional fragment with the 

 normal locus, the normal phenotype appears, though in a number of 

 cases the recessive action remains. We can express both situations by 

 saying that usually one dominant dominates two recessives, but 

 sometimes the two recessives are dominant (or part dominant) over 

 one dominant. This group of facts certainly demonstrates that a locus 

 plus its next neighborhood (the smallest extra fragments are still 

 relatively large) can have the specific action of one allele or a pair 

 of alleles independently of the rest of the chromosome, and thus 

 may be considered as a separate entity, the gene. However, no system- 

 atic quantitative study of this problem has yet been made. The 

 question to be solved is whether this group of facts means that all 

 genetic actions are based upon particulate genes. 



B. THE THEORY OF THE GENE MOLECULE 



a. Treffer theory 



It was emphasized repeatedly that "the gene" is known only 

 through a change, the mutant condition; or, more correctly, is an 

 extrapolation from the mutant locus. From this it follows that we 

 should know what a gene is, if it can be realized what the mutational 

 change means. An experimental attack upon this problem became 

 possible after Muller (1928) discovered the production of mutation 

 by ionizing radiations; and the dosage law of this effect was added 

 by Oliver (1932), Hanson and Heys (1928), Stadler (1928), and 

 others. The first theoretical analysis of the meaning of the facts for 

 the theory of the gene was made by Delbriick in Timofeeff-Ressovsky, 

 Zimmer, and Delbriick (1935). The argument consists of two parts. 

 The first is the so-called Treffer (target) theory, introduced by 

 Crowther and Dessauer (see in Dessauer, 1954) as a general theory 

 of the action of ionizing radiation upon biological systems. The idea 

 is that a single hit produces a pair of ions which is responsible for 

 all effects. Applied to radiation-produced mutation, the law of pro- 

 portionality of mutations can be expressed in a general empirical 

 equation. This is compared with the general equation covering all 

 cases of effect by a definite dose upon x out of n possible areas. 



