CHROMOSOMES IN GENES 



59 



though it is frequently forgotten — the 

 classical conception of the gene is an extra- 

 polation from three main facts: (1) muta- 

 tion, (2) the localization of mutation at a 

 definite locus in the chromosome, and (3) 

 the possibility of reshuflfling these loci. 

 From these facts the inference is drawn 

 that a particle, located at the normal locus, 

 plays a major role in the control of normal 

 development of the traits which are af- 

 fected by mutation. The process of muta- 

 tion, then, is assumed to change chemically 

 this particle, the gene, and the normal gene 

 becomes a mutant gene. Mutation, there- 

 fore, might furnish information regarding 

 the nature of the gene. Actually, the solu- 

 tion of this great problem is already claimed 

 in certain quarters : it is claimed that the 

 facts relating to the effect of radiation upon 

 mutation-rate have already solved the prob- 

 lem. Timofeeff-Ressowsky, Stubbe, and the 

 physicist Jordan repeat over and over 

 again, even in semipopular books, that it 

 has been proved that the gene is a single 

 molecule, and Kiihn has gone so far in a 

 recent presidential address as to say that 

 one of the great results of collaboration be- 

 tween biologists and physicists is the proof 

 that the gene is a single molecule. Such 

 statements, backed by the reputation of 

 well-known scholars, are most regrettable, 

 as they can be made only if the entire devel- 

 opment of genetics during the past seven 

 years is neglected. The situation is this: 

 When Muller discovered the production of 

 mutations by X-rays, and measured the 

 quantity of the effect by his ingenious CLB- 

 method (lethal mutations in the X-chromo- 

 some are recognized by their effect upon 

 the sex-ratio) he was perfectly entitled to 

 assume that he measured the rate of gene- 

 mutations. Then came Hanson and Heys' 

 discovery that the rate of X-ray induced 

 mutation is independent of the wave-length 

 but directly proportional to the amount of 

 ionization produced by the dosage. This 

 fact, confirmed by many others, together 

 with a few measurements made by Timofeeff- 

 Ressowsky of the temperature coefficients of 

 the rate of mutation furnished the physicist 

 Delbriick with the material for a theoreti- 



cal analysis. The result was that it must 

 be assumed that a single electronic hit pro- 

 duced the effect, the mutation. It was 

 further concluded that this hit produces, in 

 a single elementary process, a definite rear- 

 rangement within a molecule if a point is 

 acted upon which is capable of such a rear- 

 rangement. Therefore the proper model 

 for a gene would be a stable compound of 

 atoms, i.e., a single molecule. (At the end of 

 the paper, however, a little loophole is left, 

 saying that maybe the whole chromosome is 

 a big atomic compound with many rather 

 autonomous subgroups.) It is evident that 

 Delbriick, who himself spoke always of a 

 model and never claimed to have proved 

 that the gene is a molecule, based his analy- 

 sis upon the assumption that the basic data 

 furnished him by geneticists were con- 

 cerned with gene-mutations. But already 

 this assumption had begun to crumble. 

 Muller, himself, soon followed by many 

 others, found that X-rays produce also 

 chromatin-rearrangements. Since then it 

 has become more and more clear by the 

 work of Muller, Prokofiewa, Demerec, 

 Sokolow, and many others that the majority 

 of X-ehromosome lethals produced by 

 X-rays are chromatin rearrangements and, 

 therefore, that conclusions based upon the 

 quantitative material mentioned above ap- 

 plied to chromosome-breaks and not genes. 

 But not enough with that. It was also 

 shown that the rule of proportion to dosage 

 is true also for actual chromatin rearrange- 

 ments (Belgowsky, Muller). Finally, it 

 appeared as well in genetic experiments 

 with special methods (Muller, Bauer) as in 

 purely cytological X-ray work (Sax, Catche- 

 side, et al.) that the rate of translocations 

 produced is such a one that the maximum 

 rate of genuine gene mutations which could 

 possibly be claimed after X-radiation {i.e., 

 mutations for which a chromatin rearrange- 

 ment can not yet be demonstrated by cyto- 

 logical methods) shrinks into utter insig- 

 nificance. Today it is safe to say that the 

 predominant, if not the only, action of 

 X-rays upon the hereditary material is to 

 produce breaks in the chromosomes, with 

 consequent rearrangements of the chromo- 



