THE NATURE OF THE GENE 299 



from the facts of reverse mutation and multiple allelomorphio 

 series do not appear insurmountable to Stadler. Regarding the 

 gene, he thinks that the association of induced mutation with 

 chromosome breakage does not necessarily exclude the possibility 

 that mutations are intragenic changes, for it is conceivable that 

 some change within the gene may be the cause of the break. But 

 the identification of the induced mutations in general as a class 

 distinct from the chromosomal abnormalities is not possible in 

 the experiments with plants. Stadler, like Serebrowsky, does not 

 draw any conclusions from his interpretation of mutation 

 concerning the nature of the gene. It might be assumed that he 

 took it for granted that small deficiencies may be regarded as the 

 loss of one or a few genes, whatever they are. Serebrowsky, as 

 we have seen, had definite views about the gene (the step struc- 

 ture) which, however, were not derived from the evidence 

 regarding deficiencies. 



In a former chapter (see page 176), we discussed Demerec's 

 work with induced small deficiencies in Drosophila. Of the large 

 number that he tested, all but one were cell lethal. This excludes 

 all these deficiencies from being of the same type as recessive 

 mutations. The latter, then, must be assumed to be still smaller 

 deficiencies if Stadler's views are correct. We shall return later 

 to this problem, the significance of which lies in a different 

 direction. 



5. THE GENE AS PART OF A HIGHER UNIT 



The ideas regarding the nature of the gene, as thus far reported, 

 are based upon the assumption that the gene is an independent 

 unit, a molecule or a group of molecules, attached to the chromo- 

 some at a definite locus. The chromosome, i.e., its essential 

 part, would have to be visualized as a string of genes, com- 

 parable to a string of beads, in which the bead is a unit and the 

 string a different thing. This does not necessarily include 

 the concept that these beads, the genes, are completely inde- 

 pendent of each other. Muller (1932) has occasionally spoken of 

 intrachromosomal balance, which would mean a certain relation 

 of interactions of genes within a chromosome as different from 

 those between genes in different chromosomes. Schmalfuss 

 (1929) when discussing chemical models of genes pointed out 

 that the relative position of gene molecules might have an effect 



