Genie and Non-genic Parts of the Chromosome 87 



posed to be controlled by these blocks was checked in the presence 

 or absence of Y and extra Y material. As I mentioned previously, Y- 

 chromosomes of different origin exert a specific, rather small influence 

 upon the number of chaetae, some in the minus, some in the plus di- 

 rection. This effect is continuous and rather small in comparison with 

 that of environmental factors. The action "must" depend upon a num- 

 ber of genes, according to Mather. Since it is assumed that the poly- 

 genic blocks in different chromosomes have a similar effect, it is 

 assumed also that the heterochromatic Y-chromosome has all the prop- 

 erties of a polygenic setup: quantitative action depending upon a 

 number of genes with similar small quantitative effects, balance of its 

 polygenic blocks, and possibility of new combinations via crossing 

 over. Reasoning backward, therefore, the polygenic blocks are hetero- 

 chromatic, or all heterochromatin contains polygenes. 



It can be understood that this analysis has not met with general 

 approval. This applies to the conclusion of the existence of polygenic 

 blocks as well as to their connection with heterochromatin. Here we 

 are interested only in the second part, which was the first attempt to 

 assign a definite genetic function to heterochromatin. It has been 

 argued, since, that duplication of genes could be a method of pro- 

 ducing such polygenic blocks. The facts are insufficient for a con- 

 vincing proof of the existence of the polygenic blocks and of their 

 heterochromatic nature, though some of the conclusions concerning 

 the action of heterochromatin, apart from the polygene concept, agree 

 with those we derive from different facts. 



Rather different conclusions about the genetic function of inter- 

 calary heterochromatin have been derived by Goldschmidt (1949c) 

 and Goldschmidt, Hannah, and Piternick (1951) from their study of 

 the podoptera mutants of D. melanogaster. The very large and com- 

 pHcated body of facts may be summarized thus: podoptera (and 

 the similar tetraltera) is a group of homoeotic mutants in which the 

 wing is changed in a characteristic way into a leglike (sometimes 

 haltere-like ) structure; furthermore, other wing parts (i.e., their 

 embryonic material) are transformed into thorax. A considerable 

 number of such known podoptera types are genetically and pheno- 

 typically somewhat different. They are no simple mutants, but the 

 phenotype is based upon a number of mutant loci with different 

 quantitative effect, collaborating in a definite way. We speak, there- 

 fore, of the podoptera effect, not of simple pod mutants. Genetically, 

 pod factors are known in all chromosomes. If isolated, some have very 

 little effect; others, large effects; and together they produce a pene- 



