78 Nature of the Genetic Material 



of development like speed of cell division. This conclusion replaces 

 the old, no longer tenable idea of Muller and Painter (1932) that 

 block heterochromatin, especially the Y, is genetically inert. Of 

 course, if this were so, it would be hard to understand why the inert 

 material would be conserved in phylogeny. In a general way our 

 conclusions agree with those of Caspersson (1947) and Darlington 

 (1942), who make the more specific biochemical claim (which is very 

 difficult to prove by the facts thus far known) that the heterochro- 

 matin controls cell metabolism and especially the nucleic acid syn- 

 thesis. 



The idea of inert heterochromatin in my opinion is much abused, 

 especially in speculations of a phylogenetic type. Evolution of difi^er- 

 ent chromosome numbers in nearly related species (e.g., Crepis: 

 Babcock, 1947 a,b; Tobgy, 1943 ) is explained by one or more chromo- 

 somes becoming, in some way, heterochromatic, and, since this is 

 assumed to mean genetically inert, such an element is supposed to 

 become useless and be removed by selection. The same idea has 

 been used to explain differences of evolutionary significance in chro- 

 mosome arms or sections (see Smith, 1952). It seems to me that 

 we should be very cautious about such speculations. They are all 

 based upon Muller's proof that deletion of the large block of chromo- 

 central heterochromatin in the X-chromosome of Drosophila does not 

 influence the phenotype. Since that time so many cases — mentioned 

 or to be recorded — have been found in which addition or subtraction 

 of heterochromatin (including the X-chromosome block) influenced 

 the phenotype that the idea of "inertness" must be regarded as dis- 

 proved. 



Most of these speculations deal with loss of chromosomes via 

 inertness. But they have also been extended to the gain of chromo- 

 somes in genera in which the oldest species is supposed to have the 

 lowest number of the group. As an example of what in my opinion 

 is a perverted way of looking at the problems of heterochromatin, I 

 mention the work of Kushnir ( 1952 ) on the chromosomes of Gryllo- 

 talpa species. She speaks of heterochromatic polysomy which led 

 from a form with 15 chromosomes to a form with 19 and 23. The idea 

 is that "the heterochromatic content of these chromosomes which, 

 on one hand, was responsible for their multiplication through irregu- 

 lar segregation enabled them, on the other hand, to survive in larger 

 numbers without upsetting the genie balance of their bearers. While 

 such an addition of heterochromatin must have entailed immediate 

 effects on the metabolism of the animals, it also prepared the ground 



