Chromosomes and Genes 161 



in terms of genes and their mutations, as long as we do not inquire 

 into what a gene is and what a mutation is. The important point is 

 that there is no individual locus and no gene which mutates, but a 

 segment of a chromosome which is rather large in molecular terms, 

 which has an orderly, internal structure of a definite sequence (i.e., 

 polarized), and which may even overlap the next one; and that any 

 happening within such a segment which changes this sequence visibly 

 or invisibly appears as a mutant, all of which are allelic. If anybody 

 would now call these segments the genes, in order to save the gene 

 concept, the device would be pure semantics, for nothing is left of 

 the classic gene and its mutation. 



Sturtevant once said that anybody who holds such views does not 

 know what a gene is. What he probably meant was that a gene is a 

 chromosome section between two crossover breaks, and by that 

 definition he assumes he is in possession of the secret of what a gene 

 is. Such dicta ex cathedra touch only the semantics of the case and 

 leave the problems where they were. 



The definition of position effect which we had to use thus far in a 

 descriptive way, in terms of action of a locus near a break, now 

 becomes obsolete. The position effect is, like mutation, the effect of a 

 change in the serial order of the structural elements of a section of a 

 chromosome. The diagram illustrates some of the possibihties. 



c/2 means that instead of submicroscopic c^c^c^c^ only c^c^ is present; 

 q or 3 means that instead of submicroscopic b^b^b%^ the orders 

 b^b^b^b* and so on are found. 



This diagram and the preceding discussion make it appear as if 

 the segments in which a disturbance of order produces the position 



