INTERCLNIC AND INTRAGENIC CHANGE 



in total B content, has more Bar effect than docs ABBC/ABBC. Its 

 eyes have fewer facets. Evidently the B's are disproportionately 

 effective when they are concentrated on one chromosome. The effect 

 of their unbalance is not additive but multiplicative. This is another 

 and special example of the position effect, and indeed it was the 

 first that could be proved, since it could be done and undone so 

 readily by crossing-over. 



Thus simple additions, subtractions, and dislocations of genes 

 produce unit hereditary effects. At one time Bateson considered that 

 plus and minus differences, due to mere presence and absence of 

 determinants, were the essence of all hereditary change, the presence 

 being dominant to the absence. This led to a difficulty because mere 

 loss (or even perhaps gain) could not ultimately produce anything 

 new. Unpacking requires a previous packing. Furthermore, crucial 

 evidence against Bateson's view is that the two allelomorphs of many 

 genes (like white eye and red eye in Drosopliila) can each mutate 

 to the other. Change has often been observed in this way in both 

 directions ; and where it has been seen only in one, that is doubtless 

 sonietimes due to the frequency being low, and lower one way 

 than the other. Yet the absence could hardly be expected to mutate 

 to the presence. Unless, like the normal allelomorph of Bar, it is only 

 half an absence. Finally, as we have seen, the absence, the deficiency, 

 can be at least partly dominant. 



It is not, therefore, surprising that when most mendelian hetero- 

 zygotes are examined, for example, at pachytene in maize or at 

 polytene in Drosopliila, we find no visible differences between their 

 pairing chromosomes. Both dominant and recessive thus prove to 

 be present. There is evidently a residual class of differences which 

 cannot be shown to be due to structural change. Ultimately, of 

 course, we must assume that all changes are structural in the sense 

 of being changes in number or arrangement of some constituents. 

 Since a linear order has been established for the genes, changes 

 between genes must always be longitudinal. We can still suppose, 

 however, that genes have additional dimensions, and that lateral 

 change occurs within them. 



To put the argument in another way, the occurrence of a second 

 kind of change other than one of mere arrangement is inevitable. 

 Arrangement of units has no meaning unless there are differences 



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