no Nature of the Genetic Material 



proportionality eflFect to dose in either case. A similar group of facts 

 shows that the number of mutations differs widely according to 

 whether spermatozoa or ordinary sex cells are irradiated. Another 

 group of facts shows that the number of mutations varies according to 

 different physical, chemical, or thermal conditions to which cells are 

 subjected before irradiation. This shows that an activation or ioni- 

 zation within a sensitive volume may or may not produce a mutation, 

 and that facts are not so simple as the Treffer theory assumes. Another 

 group of facts relates to production of mutants by a hit outside the 

 genetic material as evidenced by two break deficiencies produced by 

 a single hit. Muller, who certainly knows more than anybody else 

 about all aspects of the problem and has himself ventured calculations 

 of the size and number of genes, is very cautious in regard to such 

 calculations as those reported and actually discounts them. Thus the 

 argument as to a molecular gene derived from calculations based upon 

 uncertain premises, even upon reasoning a posteriori, may be con- 

 sidered highly biased and improbable, and it does not seem worth 

 while to enter further into this topic. It is true that we read frequently 

 in texts, general books, and even in genetic or evolutionary papers that 

 the number of genes in Drosophila is around 3,000 (or any other 

 number) and in man from 20,000 to 30,000. In my opinion such state- 

 ments are gratuitous and should be regarded with the greatest caution. 

 All the subsequent discussions will bear out this statement. 



C. THE THEORY OF CHROMOSOMAL HIERARCHY 



We have come to the conclusion that the classic concept of the 

 chromosome as a gene string, after the model of the string of pearls, 

 and its development into the theory of the gene molecule cannot be 

 considered as incontrovertible facts, as some geneticists would like to 

 make us beHeve by strongly worded dicta, or by emphasizing the 

 (unknown) size of the majority who remain faithful to the classical 

 gene, or by acknowledging that the ideas of the gene are undergoing 

 a change which, however, must not be a real change. The foregoing 

 discussion of this situation was meant as a starting point for the 

 analysis of other possibilities which have presented themselves during 

 the past decades ( Goldschmidt, 1938a, 1938^^, 1944, 1946Z?, 1952a; see 

 also Mather, 1946, 1948; Whiting, in Lewis, 1951). It might be of 

 interest to report, as a kind of historical introduction, some of the 

 groping attempts to get away from the inflexible and unsatisfactory 

 classic theory of the gene. 



