Chromosomes and Genes 151 



general phenomenon and that all the conclusions we drew from it 

 can be generalized. 



The next group of facts shows that the different cases involve 

 different rearrangement breaks (which was expressed by McClintock 

 by describing Ds as a jumping locus). It further demonstrates that 

 with the position effect break constant, the other break may be any- 

 where, which may lead to special phenotypic differences, just as in 

 the above-quoted work of Raffel and Muller in Drosophila. (These 

 different rearrangements and their effects are called by McClintock 

 the different states of Ds, which again is a very confusing termi- 

 nology. ) 



The activator, Ac, usually is needed for the variegated position 

 effect to take place, but sometimes it is not required; also, it has 

 different states in different cases. The Ac effect may be localized at 

 different points. This suggests that Ac is the other break of the re- 

 arrangements, located in heterochromatin. It would act upon the 

 threshold of the Ds position effect, sometimes more, sometimes less, or 

 not at all, in different rearrangements. We could describe this, in terms 

 of Drosophila, as a position effect of a heterochromatic break (not 

 adjacent to the primary position effect break Ds), resembHng the 

 action of extra heterochromatin upon mottling in Drosophila. If a 

 strict comparison is made with Drosophila, the Ds effect would require 

 a moving of heterochromatin near the position effect locus, the pri- 

 mary cause of the variable threshold for expression of the position 

 effect. On the other hand, it is also possible that this is merely a case 

 in which variegation occurs without Ac. When Ac is needed, it could 

 be that the Ds break is purely euchromatic and therefore produces 

 only the recessive, complete position effect, while the Ac-heterochro- 

 matic condition shifts the Ds effect near or below the threshold. As far 

 as I know, such a situation has not been found in Drosophila. It would 

 mean, for example, that a typical white position effect became mottled 

 when something happened to the heterochromatin somewhere else. 

 Perhaps some of the cubitus interruptus effects recorded previously 

 could be a comparable situation. If maize had salivary chromosomes, 

 the secret of the Ac action could be unraveled. All this is so important 

 that we must discuss it in more detail; the present brief discussion was 

 intended only to make McClintock's work more understandable and 

 to fit in with the known features of position effect. 



ddd. Mottling in maize and Drosophila. — The end of the last 

 section contains the gist of the present chapter. We must elaborate it 



