CHAPTER 15 



THE GROWTH OF GENES 



Constructive and Destructive Chang^es Kinds of Chromonicres and Genes 



The Position Effect Complex Genes Tying Genes Together 



Super-Genes The Sex Mechanism 



The Limits of the Super-Gene 



In order to understand the way in which genes becomes adjusted 

 to one another within the nucleus, it was necessary for us to look 

 at the ways in which individuals were adjusted to one another 

 within the population. We found that balance could be understood 

 in its entirety only in terms of the breeding system. 



We must now take a reverse step in regard to selection. Having 

 seen how selection operates on the phenotype and, through this 

 channel, on the inter-relations of the genes, we must turn back to 

 examine its effects on the genes themselves, in their dual capacity 

 as units of recombination and of action. 



Constructive and Destructive Changes 



In general, plant and animal breeding and evolution suggrst that 

 the changes wliich are constructive in the sense of being new adapta- 

 tions are of polygenic origin. The major discontinuities used in 

 mcndelian experiment, on the other hand, are largely destructive. 

 The overwhelming majority of mutants in DrosopJiila and Antirr- 

 hinum, especially those produced by X-rays, are hypomorphic and 

 selectively negative, or at best selectively nearly neutral. 



One or two apparent exceptions are worth noting. In the liverwort 

 Marchantia, Burgeff produced a mutant, hlastopJiora, in which the 

 single three-sided apical cell was replaced by a many-celled meristem, 

 a change which £rst appears in the Pteridophyta. Similarly there is 

 the mutation found by Andersson and Gairdner in Scolopemirium, 

 by which the spores are converted into spcrmatozoids and the fern 

 promoted in its life cycle, as it were, into an animal. A third 

 mutation is the hemiradialis of Antirrhinum which restores radial 



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