Chromosomes and Genes 107 



susceptibility to reverse mutations for different loci and for inductions 

 by diflFerent chemicals ( Kolmark and Giles, 1954 ) . Thus diepoxybutane 

 immensely increases reversal at the adenine locus as compared with 

 the inositol locus. It is obvious that a test for small rearrangements 

 (see the powerful action of X rays) cannot be made in Neurospora. 

 However, in view of the fact that temperature-dependent biochemical 

 mutants are frequent in Neurospora (see Horowitz and Leupold, 

 1951 ) and that in the flour moth a biochemical mutant may still have 

 the necessary enzyme for normal action ( Caspari, 1946 ) , we might ask 

 whether the biochemical reversals wiU not find a completely different 

 explanation. I should certainly hesitate to use them for conclusions 

 upon the nature of the gene. 



Thus it looks today as though return mutation were at best very 

 rare and probably non-existent. This would mean that mutation is an 

 irreversible (or almost irreversible) event, a result which must be 

 taken into account when theories about mutation and the nature of 

 the gene are developed. We did not mention in this connection, 

 except in a short parenthesis above, the so-called mutable loci, which 

 are frequently referred to as return mutations. We shall see later that 

 this phenomenon has found a completely different explanation. It is 

 actually based upon a typical position effect, and thus constitutes a 

 strong endorsement for our skepticism. 



c. Counting the genes 



In discussions of my views on the non-existence of the corpuscular 

 gene (to be presented below), the following argument is frequently 

 heard. Many people have counted the number of genes and measured 

 their size with nearly identical results. How can something not 

 existing be counted and measured? The answer is that we can get any 

 answer to questions if the proper elements are fed into the question. 

 We can calculate the number of people on Mars if we assume that the 

 canals are dug by people and that they accomplish as many man 

 hours as we do in digging, that slave labor prevails on Mars, and so 

 on. Does the result of such a calculation prove that men are living 

 on Mars? Indeed, this simile applies literally to the counting and 

 measuring of the genes. 



We do not intend to review these calculations, which are still 

 being made (e.g., Herskowitz, 1950), and are based upon the Treffer 

 theory. If actually one hit producing ionization suffices to produce a 

 mutation, it follows that the mutation of a gene is based upon the 

 absorption of the energy of one ionization within a small volume ( the 



