Chromosomes and Genes 149 



ment containing the dominants. Therefore, a spot with the recessive 

 characters will become visible as a variegation; the size, type, and 

 extent will depend upon the time of the event and the location of the 

 break. This explains the otherwise rare, recessive "mutable gene" 

 effect. 



An additional feature necessary for the production of the Ds 

 breaks is the presence of a dominant Ac, controlling the time of 

 incidence of the breaks in proportion to the doses of Ac present. Ac 

 cannot be localized, for it seems to occupy many different points of 

 different chromosomes. It also shows different alleles and in a general 

 way acts like Ds; but the Ds break and its variants and effects usually 

 do not occur in the absence of Ac. This is so far the least clear feature 

 in the whole situation. (What Ac may mean will be indicated at the 

 end of the next paragraph and in the following interpretation.) 



The next important fact is that of a transposition of the Ds break 

 next to the locus for color C (one of the "Ds jumps"). This locus 

 behaves now (in the presence of Ac) as if C were mutated to c, and 

 this, of course, is the typical position effect of a rearrangement break 

 near a normal + locus as in Drosophila. The additional proof, possible 

 in Drosophila only in a few rare instances, namely, in restoration to 

 normal when the adjacent break is removed, was supplied in this case. 

 The new fact is that this position effect, that is, no pigment in the 

 presence of the break, assumes the character of a mutable locus. 

 (Therefore, we compared the Ds break with a secondary mutator.) 

 This means that it frequently reverts in some cells to normal or stages 

 in between, which leads to variegation, the external sign of "mutable 

 genes." It is at once clear that this phenomenon is similar in pheno- 

 typic effect to the mottled position effect in Drosophila (see I 2 C d 

 bb ) . But whereas the latter had to be explained as a variable thresh- 

 old effect of the position effect, McClintock considers the variegation 

 in maize to be dependent on a reversal, a removal of the Ds break. 

 The type of resulting variegation serves to supply further information. 

 I wonder whether a reversal is really involved, or whether this is only 

 a hypothesis. If the latter, the alternative would be a threshold effect 

 as in Drosophila. If this is true, the role of Ac would become clear: it 

 would control the threshold conditions. 



Subsequently, exactly the same features were shown for other 

 loci, giving position effect of the variegated type if next to the Ds 

 break in presence of Ac. One is Bronze (Bz), another Sh (normal 

 versus shrunken endosperm). However, some "mutable loci" arose in 

 other experiments which did not require Ac for their action (meaning 



