THE GROWTH OF GENES 



There are attached to the chromosomes two special types of 

 structure which fall into a class by themselves, whether they arc 

 regarded as chromomcres or as genes. The nucleolar organizer and 

 the centromere can both be broken into parts less effective than the 

 whole but nevertheless having the same kind of effect. The organizer, 

 which McClintock broke by X-rays in maize, gives two organizers 

 each with a smaller nucleolus. The centromere, which splits into 

 two crosswise by misdivision in univalent chromosomes, gives two 

 daughter centromeres, each terminal and each less effective than 

 the whole though usually capable of carrying its single arm to the 

 pole. The broken centromeres show no other defect than the 

 customary non-division of a broken end gene which leads to the 

 formation of isochromosomes. Both the centromere and the 

 organizer, therefore, depend for their integration on the mere 

 multiplying of small simple elements. 



The genetic counterpart of the cytological evidence lies in the 

 distinction between the different degrees of strength and specificity 

 in the actions of differences in polygenes and in major genes. 

 Experiments with the X and Y chromosomes in Drosophila have 

 now shown that the heterochromatin contains only polygenes, 

 while the euchromatin most likely contains both. This is only 

 to be expected on the evolutionary view. The heterochromatin 

 contains only simple elements and can act only in a simple way. The 

 euchromatin contains complex elements which can act, at least in 

 some stages of development or in some relationships, as integrated 

 wholes, although in others they may act as disintegrated parts. 

 Further, in this latter capacity the genes of the euchromatin influence 

 the same processes of development in the same way as the genes of the 

 heterochromatin: they act as members of the same polygenic 

 systems. We are bound, therefore, to suppose that they are similar 

 elements, the difference being that in the euchromatin they are 

 organized so that complexes of dissimilar ones can also act together 

 in the integrated way by which we recognize major genes. 



The Position Effect 



The process of integration of genes is indicated by the position 

 effect. Two genes near one another on the chromosome can influence 

 one another's effects. This influence could be due to an interaction 



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