Chromosomes and Genes 127 



that position effect has come into its own, other examples will be 

 found. 



The two prerequisites for position effect and its discovery are 

 as follows: (1) a rearrangement can be diagnosed genetically and 

 (or) cytologically; (2) in the neighborhood of a break a locus is 

 found which, if mutated, produces a recognizable phenotypic effect. 

 (The cases discussed above stand excepted; namely, those in which 

 a dominant position effect is possibly produced by the arrangement 

 itseff, without a nearby locus which acts similarly when mutated. 

 A proof for the existence of this type is hardly possible.) The rec- 

 ognizable effect may be a distinct phenotype or it may be a physio- 

 logical condition recognized only as different vitality, lethality, 

 sterility, and so on. The first prerequisite makes it almost impossible 

 to prove position effect directly in such materials as mammals and 

 many plants. The second prerequisite makes negative statements on 

 position effect of doubtful value. In addition, it must be recognized 

 that it is possible that chromosomal structure can be such that breaks 

 and mutable loci are never or rarely adjacent or neighboring. If, 

 for example, easily breakable sections of chromosomes (known to 

 exist) are relatively free of mutable loci which may be crowded in 

 a poorly breakable section, position effects (i.e., visible ones) will 

 be rare or absent. In Stadler's X-ray experiments with cereals, many 

 rearrangements and hardly any "point mutations" were produced. A 

 chromosomal structure as just postulated would account not only for 

 such facts (under my interpretation of point mutants as position 

 effects of ultramicroscopic arrangements) but also for an absence of 

 position effect. If it is found that numerous gross rearrangements in 

 plants do not show any visible position effect (forgetting for the 

 moment the physiological effects that have not been checked), the 

 reason may be a chromosome structure as described. McClintock's 

 work shows that if the experimental production of breaks adjacent 

 to known loci succeeds, position effect is always found. Thus the 

 positive results alone have decisive value and we may say with 

 certainty that rearrangement breaks always produce position effects, 

 though they cannot always be seen. Therefore, position effect can 

 be called a ubiquitous standard phenomenon of genetics with which 

 any theory of the genie material must reckon, or, as I believe, from 

 which it must be derived primarily. 



The other important statement is that position effect shows all 

 the characteristics of point mutation. It is well known that many 

 mutants which were thought of as point mutants turned out on 



