THE GENE 28-7 



of blocks of regions, each of which is devoted to the synthesis of a 

 specific gene -product, some of which may even overlap. If cross- 

 overs which tended to break up these specialized regions occurred 

 only rarely there would be little interference with gene action by 

 crossing-over. The deficiency revealed as a Mendelian gene would 

 behave as a single entity on segregation, although it comprised only 

 a single step in a series of reactions. 



THE LINEAR CHROMOSOME AS THE SEAT 

 OF ORDERLY REACTIONS 



Some mechanism for carrying out orderly reactions is essential 

 in a cell whose cytoplasm is almost completely fluid. If the chromo- 

 some be the seat of all orderly reactions, the necessity for imposing 

 orderliness upon a fluid cytoplasm disappears, and the linear structure 

 of the chromosome takes on deeper significance. According to this 

 view, syntheses requiring a series of steps in which one reaction 

 follows another occur on the chromosome. When a substance such 

 as galactose appears in the cell, one of these **chain" reactions re- 

 sults in the final production of galactozymase, if the catalysts essen- 

 tial for its completion are all at their proper loci. When the syn- 

 thesis is in full swing, galactozymase deals with galactose in the cy- 

 toplasm, without any necessity for orderly arrangement of the com- 

 ponents of the reaction. We may look in the future for short sequences 

 of "genes" along the chromosome which have significance in terms 

 of the reactions that are involved. At the present time, these se- 

 quences are not obvious, both because the total number of "genes" is 

 much greater than the standard methods are capable of discovering, 

 and because our knowledge of the chemistry of the synthetic reactions 

 occurring in cells is altogether too inadequate. Present methods 

 probably give lower apparent numbers of "genes" because most seg- 

 regating irregularities are repaired in the heterozygous condition. 

 Actually, the concept of the "number" of genes may be relatively 

 meaningless and probably only a small fraction of the actual "hetero- 

 zygosis" is experimentally realized, and the "number" of "genes" 

 may be many hundreds or thousands of times greater than the largest 

 current estimates, for only a few molecules might be sufficient to 

 constitute a given "gene" and blocks of the chromosome may comprise 

 a continuum, with adjacent "genes" occasionally overlapping. 



The classical view of the gene was based on (1) the demonstration 

 by genetical experiments that certain characters were inherited as 

 units, and (2) the demonstration of individual chromeres in the chro- 

 mosomes by cytological methods. The final supposedly "crucial" ex- 

 periment was the demonstration that the bands in the salivary chro- 

 mosomes could be specifically associated with unit characters. How- 

 ever, the view that this constituted a "crucial" experiment is due to 



