126 Nature of the Genetic Material 



Thus I could show (1945fl) that Blond may be a position efiFect for 

 the nearby silver locus, as is also the dominant pointed (svr^'). Un- 

 fortunately, it is hardly possible to produce a direct proof that this 

 type of position effect per se exists. 



bb. Frequency and types 



In order to derive conclusions about the nature of the genie 

 material from the phenomenon of position effect, we must first con- 

 vince ourselves that position effect is a general and typical feature 

 and that in principle it is not different from the phenomenon of 

 point mutation, which we wish to consider as an ultramicroscopic 

 position effect. Fifteen or so years ago it was still possible for some 

 geneticists to declare that position effect was such a rare and special 

 phenomenon that it would be faulty to generalize from its occur- 

 rence, and I was severely reprimanded for doing so (by Babcock, 

 Jenkins, and Stebbins, and by Plough). Today nobody can doubt 

 that the effect is a general and widespread phenomenon, and has just 

 the significance for the theory of the gene which I have attributed to 

 it for two decades. 



Most of our knowledge is still derived from Drosophila work. 

 This will probably remain so, because the salivary chromosome tech- 

 nique permits exact location of rearrangement breaks within ap- 

 proximately one one-thousandth of the chromosome length and 

 because the genetical localization of hundreds of loci makes a huge 

 material available. However, McClintock's work (1951) has opened 

 maize genetics to a large-scale study of position effect, which clearly 

 is as frequent in maize as in Drosophila if the proper setup permits 

 recognition of it. We still hear that, especially in plants, innumerable 

 gross rearrangements are known without a position effect being 

 noticed. According to an early observation by Muller and Altenburg 

 (quoted above), some effect upon viability, fertility, and phenotype 

 is found in all gross rearrangements of Drosophila. It is expected, 

 but rarely checked ( see, however, Jones' work, 1944 ) , that physiologi- 

 cal effects exist also in plant materials, including all the translocations 

 studied by Anderson (1935-1941) in maize and supposed to have 

 no position effect, a plant which is now a prototype of position 

 effect. A scrutiny of the literature will bring to light many more 

 such cases. Lamprecht (1949) described a "mutant" in Visum, al- 

 ways "accompanied" by a translocation, which, of course, means 

 a position effect. Hagberg and Tjio (1950) found that the mutant 

 erectoides of barley is based upon a homozygous translocation. Now 



