144 Nature of the Genetic Material 



protection from the chromonema. I cannot find any explanation of 

 how this would lead to point mutation, or a statement that point 

 mutations are submicroscopic rearrangements, 



I consider as an example of the same or similar type a finding 

 of Sturtevant (1939), who, after crossing two Drosophila species, 

 found a large increase of both rearrangements and point mutations. 

 These species have rather different intimate structures of the salivary 

 chromosomes, and in the hybrid synapsis is correspondingly upset. 

 It is not diflBcult to visualize that these tensions produce chromosome 

 breaks and rearrangements mechanically, just as stickiness does. If 

 the rate of point mutation also increases simultaneously, the same 

 conclusion is in order: they are invisible rearrangements. 



I am inclined to look in the same way at another group of 

 facts, the so-called mutator effects, which are very difficult to explain 

 otherwise. It is known that different strains of Drosophila have 

 different tendencies to mutate. Sometimes an unusually high muta- 

 tion rate is found, based upon a genetic condition in a chromosome 

 which has been described as a mutator gene (Demerec, 1937; Neel, 

 1942&; Ives, 1949). We may distinguish three types. The first is the 

 typical mutator effect. The second is a rather rare occurrence which 

 thus far cannot be further analyzed and is, therefore, simply dis- 

 counted by many geneticists, but there is no doubt of its authenticity. 

 In all the observed cases ( Goldschmidt, 1929fo, 1939; Plough and 

 Holthausen, 1937; for details see Goldschmidt, 1939) a kind of mass 

 mutation occurred, producing suddenly in an otherwise normal line 

 large numbers of highly diversified mutants. If such an event could 

 once be analyzed properly, after early recognition, important results 

 might be expected. The third type is also of the mass mutation type, 

 with the addition that it seems that a mutator locus can be located 

 (Mampell, 1945, 1946). Furthermore, in one of these latter events 

 most of the mutants were Minutes, known to be, as a rule, deficiencies 

 (see 12 C d dd). 



I think that these three groups of facts are all based upon the 

 same thing. The mutator is a "sticky" section in a chromosome, which 

 adheres variously to other chromosomes with the result that pieces 

 are pulled out or breaks produced which result in any known type 

 of mutation. Mampell's specific mutator action producing Minutes 

 strongly points in this direction. It is to be hoped that an analyzable 

 case may crop up or a method be found to produce it experimentally. 

 I once beheved (Goldschmidt, 1929fo) that temperature shocks were 

 such a method, but this turned out to be unproved (Goldschmidt, 



