86 Nature of the Genetic Material 



It does not seem that the existence of standard mutants within 

 intercalary heterochromatin has been demonstrated ( or can be demon- 

 strated). There are indications (Griffen and Stone, 1940) that in- 

 tercalary heterochromatin breaks producing position effects lead to 

 variegation just as do breaks involving block heterochromatin. If 

 this is confiimed, the intercalary would have one important feature 

 in common with the chromocentric heterochromatin. We have already 

 mentioned the greater breakability of this heterochromatin which 

 must result in more deficiencies and also more position effects in the 

 neighborhood of heterochromatic blocks. This would make eventual 

 heterochromatic mutants suspect. At present it is safe to say that 

 intercalary heterochromatin is genetically different from euchromatin 

 and also from block heterochromatin, with which it has, however, 

 some features in common. 



The first to ascribe a special genetic function to the intercalary 

 heterochromatin was Mather (1941), who <ierived his conclusions 

 from selection experiments with the number of abdominal bristles 

 in Drosophila. From the way the successful selection was running, 

 especially the sudden jumps after periods of constancy, he concluded 

 that the multiple factors controlling the character are present not 

 only in a balanced condition which permits plus and minus shifts, 

 but that they are packed tightly in blocks, which he called polygenic 

 blocks, within which crossing over can occur, leading to a sudden 

 change in balance. These polygenic blocks are spread over all chromo- 

 somes and are important as an always available source of adaptive 

 selection. 



Looking for an explanation of these assumed polygenic blocks, 

 Mather reasoned in the following way: supernumerary chromosomes 

 in plants are not entirely dispensable, as they remain in the cells, 

 though theoretically they should be sorted out during cycles of di- 

 vision. They must, therefore, betray a kind of balanced condition 

 favored by selection. This means, he thinks, that they take part in 

 a balance of polygenic action. They do not contain major genes and 

 are considered to be heterochromatic. Because they are, nevertheless, 

 partly dispensable, genes with similar action must be contained in 

 the ordinary chromosomes. These latter contain, as derived above, 

 groups or blocks of polygenes, balanced to insure both genetical 

 stability and flexibility, the balanced polygenic blocks. By analogy 

 with the supernumeraries, also subject to polymeric balance, it may 

 be assumed that these polygenic blocks are located in the heterochro- 

 matic sections. Therefore the phenotype (abdominal chaetae) sup- 



