Chromosomes and Genes 175 



ally speaking, these mutants T, t'^, and Ki affect the notochord-meso- 

 derm material in development, but at different times and in different 

 ways. Thus one chromosomal section is concerned in the determina- 

 tion of one basic feature of development. Actually, many more such 

 loci are known in the same region of the chromosome and no other 

 kinds of mutants. Obviously, these facts are of the same order as 

 the others discussed before, and must be explained in the same way. 



In this chapter I have refrained from going into the detailed 

 hypotheses of pseudoallelic action which attempt to explain it in 

 terms of genie actions and the primary and secondary substances 

 produced by such actions. The gist of these ideas, which derive more 

 or less from Stem's explanation of position effect, is that nearby loci, 

 originated by duplication, may have assumed consecutive functions 

 in a series of synthetic steps. Since these theories do not have so 

 much to do with the nature of the gene, under discussion here, as 

 with the genie action at the chromosomal site, they will be discussed 

 in a later chapter. But it might be asserted that the mere possibiHty 

 of construing such theories is in itself a proof for Lewis' phylogenetic 

 theory of pseudoallelism. The quotation from Pontecorvo demon- 

 strates that such a conclusion would be unwarranted. I realize that 

 some workers may think that those action hypotheses, couched in 

 biochemical terms, are a necessary part of the phenomenon. Chov- 

 nick and Fox (1953) express this by saying that it is wrong "to use 

 phenotypic and physiological observations as a basis for conclusions 

 regarding transmission phenomena. The practice of operationalism 

 as defined by Bridgman and used by Wright reconciles the apparent 

 contradictions between the physiological and transmission aspects of 

 pseudoallelism." 



Whatever this may mean (see also Stadler, 1954; and discussion 

 in I 3 C), I prefer to look at the problem in general terms of the 

 theory of the gene and position effect, in order to unify the genetic 

 and cytogenetic facts without being bound to particular ideas on 

 genie action. Thus I shall discuss genie action apart from the pres- 

 ent topic. The discussion confirms my opinion that the explana- 

 tions for the entire group of facts in terms of the classic gene require 

 special assumptions for every part of the phenomenon — for the 

 similarity but also the diversity of the individual alleles, for their 

 different actions whether in the same or different chromosomes. My 

 interpretation takes care of all aspects of the problem without special 

 hypotheses ad hoc and, in addition, links this problem closely with 

 the understanding of position effect, thus integrating the facts in a 



