Chromosomes and Genes 153 



Thus the great lines of the work on Drosophila and that on maize 

 are completely parallel, and the theoretical conclusions about the gene 

 are the same in both, as reahzed also by McClintock. However, there 

 is still one very important difficulty. In maize the variegation may 

 affect the cells of the germinal line which produce normal offspring 

 as if a mutation of the recessive to the dominant (i.e., a reverse 

 mutation) had occurred, though this is not always so (Stadler). It 

 always appeared strange ( Emerson, Rhoades, and others ) that reverse 

 mutation, which is usually so rare, if it exists at all, would be so 

 frequent a phenomenon in this case and in the numerous parallel 

 ones in plants, upon which an extensive literature is available. This 

 indicated to me in the past that a different interpretation of the maize 

 variegation would be found one day, an opinion frequently expressed 

 in the course of years. I think that the new interpretation can be 

 presented now. Take as example an aleuron color. In the kernels 

 studied by McClintock, all cells containing the normal C or A locus 

 were made to act as a recessive by the adjacent rearrangement break. 

 This effect varied from almost colorless to normally colored under the 

 influence of internal (genetic) and external conditions. How can 

 these, if found in the germ line, remain true breeding? 



We must first state that the "variation" in the mottling effect and 

 its counterpart in maize variegation, both caused by weakening of the 

 position effect, may be one of different grades, demonstrated in 

 Drosophila by different eye colors between white and red (see Stone 

 and Griffen), and in maize by different color conditions in the spots, 

 described by McClintock as allelic effects (what a source of error in 

 this terminology, in itself correct!). In addition, we may have an 

 all-or-none effect (either white or red), probably, ff not certainly, de- 

 pending upon the genetically controlled threshold conditions (see I 

 3 C c bb; and Goldschmidt, 1950!?). In discussing the relations 

 between heterochromatin and sex determination, we have drawn 

 attention to the close parallel between the variegated position effect 

 and the variegated effect in some organs of intersexual moths ( Gold- 

 schmidt, 1950b; Seiler, 1949). We pointed out that here a phenotypic 

 effect ( female or male structure, color, etc. ) is based upon a develop- 

 mental alternative within the intersex with its disturbed genie balance. 

 The exact parallel is the position effect, an alternative of action at a 

 locus, producing either the normal condition or, in the presence of a 

 break, the mutant type. In those intersexes, two possibilities are given: 

 either an all-or-none effect occurs, that is, the organ (e.g., wing) of an 

 intersex is all female or all male; or the all-or-none effect is bound to 



