HETEROCHROMATIN AND POLYGENES 



of such a system have small, similar and supplementary effects, 

 because, we may presume, they have less differentiated products and 

 less elaborate action. 



The simpler action of these genes at once recalls the simpler 

 products of heterochromatin as seen in the cell. The relationship is 

 brought closer when we fmd that heterochromatic chromosomes, 

 hitherto regarded as inert, affect quantitative variation. 



In a diploid millet, supernumeraries, which are lost except in the 

 stem, lead to extra mitoses in the pollen. The larger the number of 

 supernumeraries, the larger the proportion of pollen which is killed 

 in this way. Nor is this an isolated example, for supernumeraries are 

 now known to affect the phenotype of a number of grasses and 

 cereals in a quantitative way. Quantitative effects are also produced 

 by the Y chromosome in Drosophila melanogaster. Y chromosomes 

 from different strains vary in their effects on the numbers of bristles. 

 But the greater part of this variation can be destroyed by association 

 with a common X chromosome partner. It thus seems that genes 

 in the Y, affecting bristle number, can be exchanged with 

 corresponding genes in the X. And, since only the heterochromatic 

 portion of the X lies in the pairing segment and is homologous with 

 part of the Y, we can see that X-borne, as well as Y-bome, hetero- 

 chromatin is active in this way. The crossing-over that would account 

 for such exchange has, as we saw, been cytologically inferred from 

 the occurrence of reciprocal chiasmata between their pairing 

 segments. 



The heterochromatin is thus active; it is variable in its activity; 

 and this variability is divisible by crossing-over. In other words it 

 is composed of genes similar in linear organization to the major 

 genes, but differing in kind or order of physiological effect. Indeed 

 the smallness of their individual effects implies the largeness of the 

 number of these polygenes. 



We can now recognize two types of gene and two types of 

 chromatin. The major genes occur only in euchromatin and hetero- 

 chromatin contains only polygenes. Euchromatin may well also 

 contain polygenes, perhaps in the euchromatin proper or perhaps 

 lying in segments not distinguishable as heterochromatin owing to 

 their small size or for some other reason. This is all the more 

 probable since, as we saw, many plants and animals have no 



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