GENES, MOLECULES AND PROCESSES 



less pronounced, effects. But these effects should include, as they 

 do, an influence on the other kind which gives the more complex 

 products and should have the more specific and elaborate effects. 

 Secondly, we must expect the actions of genes to depend on each 

 other and on the varying character of the cytoplasm as reflecting 

 their earlier action. Differentiation will depend to some extent on 

 the limitations of diffusion within the nucleus and the cell. Finally, 

 we must expect that the cytoplasm as well as the nucleus will 

 contain self-propagating particles, although they are likely to be 

 subordinate to the nucleus in general since they have not the same 

 powers of orderly reproduction and transmission that are given by 

 the giant chromosome molecule. 



Heterochromatin and Polygenes 



The difference between genes in heterochromatin and in 

 euchromatin is in fact made clear in several ways. As we saw, the 

 Y chromosome in Drosophila and the supernumeraries in plants, 

 both heterochromatic, have few or no drastic effects. The super- 

 numeraries are not even indispensable. And none show mutations 

 of the sharp and specific kind that are used in mendelian experiments. 

 These principles apply whether the heterochromatin composes the 

 whole chromosome, or, as in the X o£ Drosophila, a part of it. On 

 these grounds, wherever it has been found, heterochromatin has 

 been described as inert. Now, however, we have two means of 

 demonstrating its peculiar activity, hi maize many cultivated 

 varieties regularly have supernumeraries which vary in number 

 from plant to plant. Yet these chromosomes often have defective 

 centromeres and so tend to be lost at meiosis or even at mitosis. 

 Their maintenance must therefore depend on a selection which 

 favours increase in number of chromosomes, and this can come 

 about only if these chromosomes actively affect the character, the 

 phenotype, of the plant. The same principle applies to super- 

 numerar)' fragments of the X chromosome found up to the number 

 of 14 in the bed bug, Cimex lectularitds, as well as in many other 

 Heteroptera. 



The second means of approach is through the polygenic systems 

 which we have seen to govern continuous variation. The members 



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