76 Nature of the Genetic Material 



certain developmental features which control the expression of some 

 mutants of Drosophila. In the two cases mentioned it was suspected 

 that the mutants themselves were of heterochromatic nature some- 

 how controlled by properties of intercalary heterochromatic blocks, 

 though the detailed interpretations of Mather and Goldschmidt are 

 rather different. Actually, it seems that some rule is hidden behind 

 these facts. Thus it is known that the expression of Minute mutants, 

 much suspected to be of heterochromatic nature, is strongly influenced 

 by the absence of a Y-chromosome. Another case is that of the mu- 

 tant "sparkling" in the fourth chromosome (L. V. Morgan, 1947), 

 which produces irregularities in eye structure that may be called a 

 variegation. No position eff^ect is involved, but there is a suspicion 

 that the locus is heterochromatic. The phenotype is strongly influ- 

 enced by the quantity of heterochromatin. More heterochromatin 

 produces a decrease of the eJBFect; less heterochromatin, an increase 

 of mutant action, according to detailed studies. In this case both 

 Y-chromosomal and X-chromosomal heterochromatin were tested. 



This group of facts suggests a specific influence of the quantity 

 of chromocentral heterochromatin upon development of mutant char- 

 acters suspected of being based upon heterochromatic loci or euchro- 

 matic ones imbedded in heterochromatic sections. 



In a general way non- Y-chromosomal block heterochromatin acts 

 hke Y heterochromatin (see L. V. Morgan, 1947). Besides the 

 Y-chromosome, the major heterochromatic, chromocentral block is the 

 big block near the proximal end of the X-chromosome. It was sup- 

 posed that its absence or duplication was without genetic effect 

 (inert region of Muller and Painter). But it has been shown since 

 that duplications of the heterochromatic block in the X-chromosome 

 act Hke an extra Y (Noujdin, 1936); further, that deficiencies of the 

 same region (Khwostova, 1939) enhance the variegation position 

 effect. Heterochromatic regions in other chromosomes, especially the 

 second, have been tested, and it was found that their deficiency has 

 the same action as the absence of Y ( see Morgan, Schultz, and Curry, 

 1940, 1941 ) . The facts of rather different type agree well in a general 

 way, though, thus far, it is not clear what type of action is involved. 

 Nevertheless, it is possible to derive some notion from the variegated 

 position effect. Here extra Y's in normal flies make the eye color 

 mottled. In chromosome rearrangements witli variegated effect, ad- 

 dition or subtraction of the Y or its parts affects the degree of 

 variegation. In general (see Schultz, 1947) we may say that the 

 more heterochromatin the less variegation and vice versa. Further 



