Genie and Non-genic Parts of the Chromosome 17 



examples of the action of chromocentral heterochromatin upon de- 

 velopmental features can be found in the reviews and papers quoted. 



bb. Genetic functions? Chromosomal breaks 

 and heterochromatin 



In order to derive general conclusions from this array of mixed 

 facts we return to the podoptera case. Here a few more facts are 

 known which can further an interpretation. The most important are 

 the following (Goldschmidt et at, 1951, 1952Z?). The mutant Beaded 

 (Bd) in the third chromosome enhances the action of the pod factors. 

 One allele at the Bd locus was found which enhances the penetrance 

 of tetraltera (belonging to the podoptera group) almost thirtyfold. 

 In pod K only males show the phenotype, and females only when 

 they have an extra Y ( L. Kellen Piternick in Goldschmidt et al, 1951, 

 1952Z?), but Beaded is able to replace completely the extra Y in 

 pod K females! The situation here can only be that the pod action 

 is very narrowly timed in regard to the developmental processes. 

 (This can be expressed also in terms of a very narrow threshold zone 

 for determinative action.) In the male developmental system the 

 proper threshold is surpassed. In the very different female develop- 

 mental system (actually different in timing and speed of many de- 

 velopmental processes ) the threshold is not reached. Beaded, an other- 

 wise known enhancer of podoptera penetrance, acts upon this system 

 in the female by pushing the decisive determinative processes above 

 the threshold of action:. Y heterochromatin has the same effect, which, 

 however, may vary quantitatively and even in direction with different 

 Y's and different genotypes of podoptera, which includes, then, en- 

 hancement or inhibition. In a general way, these effects must act as 

 steering specific mutually attuned or balanced growth processes, 

 which result in reaching, not reaching, reaching only partly, or more 

 than sufficiently reaching, the threshold level. Differential growth 

 processes are ordinarily linked to the speed of cell division. Thus the 

 genetic facts known about chromocentral chromatin lead to the same 

 result as the cytological study of heterochromatin in chromatin dimi- 

 nution: they show an influence upon cell division and cell growth. 

 (Other generalized chemical effects were mentioned above when 

 studying the sterility factors in the Y-chromosome. ) 



These conclusions emphasize a genetic function of the hetero- 

 chromatin which is different from that of the euchromatin with its 

 specific mutant loci. We might call it a generalized function, not 

 affecting a definite, local determination process but a general feature 



