Genie and Non-genic Parts of the Chromosome 85 



deserves to be mentioned (Pontecorvo, 1944), that heterochromatin 

 generally is the result of duplications of chromomeres resulting in 

 an aggregation of chromomeres of similar function (an idea which 

 could also be paralleled by Mather's blocks of polygenes). Thus far 

 such notions can only be registered. The Minutes, however, are very 

 special occurrences. Many dozens of Minutes are known (see Schultz, 

 1929), distributed over all chromosomes with a predilection for 

 chromocentric regions. All of them are dominant, homozygous lethal 

 mutants with similar pleiotropic effects upon bristles, body size, 

 wing structure, and genitaha. Many of them are known to be de- 

 ficiencies and are also known to react to extra quantities of Y-chromo- 

 some. In addition, they interact specifically with the Beaded locus 

 ( Goldschmidt, 1949a), making Bd 100 per cent dominant instead of 

 plus or minus 5 per cent. It is known (Goldschmidt, Hannah, Piter- 

 nick, 1951) that Bd is able to replace the action of Y-chromosome 

 material. From such facts we derive the suspicion that Minutes are 

 changes, actually quantity changes within the intercalary hetero- 

 chromatin, all of which have a similar effect because of the general- 

 ized action of heterochromatin. It is very difficult to prove such an 

 interpretation, which thus remains a hypothesis. 



dd. Genetic function of intercalary heterochromatin 



The decisive question is now, Does intercalary heterochromatin 

 have a genetic effect? This would be demonstrated if it is found 

 capable of changes that can be compared to euchromatic mutation, 

 though less typical. The first question would be whether hetero- 

 chromatic mutation is altogether an erroneous assumption based on 

 overlooking the fact that heterochromatic sections contain standard 

 mutants of the same type as those in euchromatic sections. While 

 discussing the chromocentric heterochromatin, we have emphasized 

 the possibility of standard mutants within block heterochromatin. 

 We came to the conclusion that such loci as were proved to be 

 present in heterochromatic sections are to be regarded as small inter- 

 calated euchromatic sections. Naturally the same thing might also 

 occur vdthin intercalary heterochromatin, though it would be diffi- 

 cult to prove on account of the difficulty of exact dehneation of the 

 intercalary sections. If the Minutes are heterochromatic mutants, they 

 demonstrate that such mutants are rather different from euchromatic 

 ones, since all Minutes have approximately the same effects, which 

 in a general way may be described as interference with growth 

 processes. 



