Chromosomes and Genes 183 



cases have been reported which point in this direction. Some of 

 them certainly belong to the group of pseudoallelic sections (see 

 review by Komai, 1950), while others will probably fall under the 

 head of the present discussion. Unfortunately, decisive tests are diflB- 

 cult in most organisms, and therefore it is only pointed out that 

 Dunn and Caspari's (1945) T factors affecting the mousetail (see 

 also Dunn and Gluecksohn-Waelsch, 1952), and Nabours' (see 1950) 

 finding that all the many mutants affecting the color pattern of the 

 grouse locust are located in a definite section of one chromosome, 

 probably belong to the phenomenon under discussion. (See later 

 chapter on pleiotropy.) 



At this point what looks like a serious objection should be 

 registered. It is known that a normal or a mutant locus when trans- 

 posed into a new position in a different sector of the same or a 

 different chromosome still acts as before. For the normal locus this 

 has been discussed as the covering effect. Of course we are excluding 

 here the presence of a position effect break near the locus in 

 question, which would result in a reversal of its action. If the normal 

 or mutational functioning of a locus in a different setup of chromo- 

 somal pattern is a general fact, as it seems, how does this fit into 

 the scheme of the hierarchy of fields? It must be realized at once that 

 such transpositions involve a rather large segment of a chromosome 

 (measured with a salivary chromosome yardstick). The majority of 

 the mutants studied in such cases are those involved within the 

 segments of mutant action, segments which are relatively so small 

 that they remain intact in such transpositions. Thus, as far as the 

 segment and its pattern, or changed pattern in the mutant, are con- 

 cerned, nothing has changed. Only the chromosomal actions which 

 are bound to the larger fields should be different in such transposi- 

 tions and translocations. It may be safely assumed that the larger 

 fields act upon more basic (i.e., earlier) developmental processes, 

 while the smaller segments are concerned with the control of late, 

 more or less superficial features of development. Hence we would 

 expect that, in most transpositions, and so on, of larger segments, the 

 usual effects would not be found, as with small segments, but we 

 should rather expect abnormalities or lethality. It is true that the 

 ill effects of heterozygous or homozygous translocations may often be 

 due to position effects. However, when major damage like lethality 

 is found, we may attribute it to the causes just stated: change of 

 pattern of larger sectors, that is, fields of action. It has never been 

 explained why homozygous translocations are so frequently lethal. 



