82 Nature of the Genetic Material 



matic regions genetically as well as in the salivary chromosomes, as 

 far as possible. Now all these mutants behave like any other mutant; 

 hence the conclusion could be drawn that heterochromatin differs 

 from euchromatin genetically only in containing very few mutant 

 loci (genes). This is not very satisfactory in view of all the special 

 features recorded for chromocentric heterochromatin. 



One may be inclined to assume that all these loci are small 

 sections of euchromatin intercalated into the heterochromatin. In 

 favor of this assumption is the fact that the heterochromatic block of 

 the X-chromosome is represented by one regular band in the salivary 

 X (see, above, Muller; I 2 C d aa) and, further, that the Y may be 

 represented by a few bands attached to the chromocenter, which 

 must contain a bobbed allele, bobbed being such a "heterochromatic 

 locus." It is maintained (see the reviews, work mostly by Schultz) 

 that such heterochromatic loci react differently to extra heterochro- 

 matin (an additional Y) as follows: while variegation of euchromatic 

 loci is reduced by an extra Y, this is not true for some heterochromatic 

 loci. It is claimed that this is what is expected, though I am not 

 convinced of the logic of the argument. 



Actually, we know that in podoptera an extra Y may act as an 

 enhancer or as a reducer of penetrance in different genotypes, 

 including the two sexes, and the influence of an extra Y on mottling 

 is clearly the same thing as its influence upon penetrance in podoptera, 

 requiring the same explanation. As so-called heterochromatic mutants 

 also are known for which an intermediate action of extra hetero- 

 chromatin is claimed, it seems to me that there are no heterochromatic 

 loci in the strict sense of the term, but only euchromatic loci sur- 

 rounded by heterochromatin, which, like other such loci, may react 

 in different directions upon extra heterochromatin, as well as upon 

 breaks in the heterochromatin. This agrees also with the general 

 interpretation of heterochromatic function given above. If hetero- 

 chromatin exercises a general, quantitative action upon developmen- 

 tal and cellular processes, there is no reason why this must always 

 go in one direction. It is more probable that it may be positive, 

 negative, or anything in between according to the specific genetic 

 milieu in which it acts. 



Recent work by Baker (1953) seems to agree very well with 

 these conclusions. He studied a variegated position effect for the 

 peach locus in Drosophila virilis, which is located within chromo- 

 central heterochromatin near the centromere of the fifth chromosome. 

 Variegation is produced (in a large material) by placing foreign 



