206 



the same time in the same cell. This 

 latter conclusion rests on the observa- 

 tion that mutations occur concomit- 

 antly at two or more /^c-controlled 

 mutable loci when these are present 

 in the same nucleus. The similarity in 

 the type of inheritance and the be- 

 havior of Ds and Ac has been indicated 

 above. Another similarity is that 

 changes in state, loss or transposition 

 of Ac occur at the same time that 

 changes take place at the Ac-con- 

 trolled mutable loci. It would appear 

 that the changes in the physical prop- 

 erties of the specific inhibiting chro- 

 matin at the mutable loci and at Ac 

 itself are of the same nature, and that 

 all are expressions of the primary gene- 

 tic action of the material composing 

 Ac. It is suspected that Ds and Ac are 

 composed of the same or similar types 

 of material. The possible composition 

 of this material will be considered 

 shortly. 



The study of Ac and the ^^-con- 

 trolled mutable loci has made it possi- 

 ble to interpret the many patterns of 

 variegation exhibited by mutable loci. 

 The variegated pattern is an expression 

 of the time and frequency of occur- 

 rence of visible changes in the pheno- 

 type. The frequency of appearance of 

 a visible mutation need not reflect the 

 frequency of the events that occur at 

 a mutable locus, as the study of c-7fil 

 has clearly revealed. The visible muta- 

 tions reflect only the frequency of one 

 or several particular consequences of 

 one primary type of event occurring 

 to the inhibiting material adjacent to 

 the affected gene. The changes in state 

 of this inhibiting material that arise as 

 one of the consequences of the pri- 

 mary event, lead to changes in the 

 relative frequency of the consequences 

 of this event when it again occurs in 

 future cell and plant generations. Such 

 changes in state are reflected either in 

 increases or decreases in the relative 



MCCLINTOCK 



frequency of appearance of visible 

 mutations. The study of Ac has indi- 

 cated the nature of the control of the 

 time when the mutations will occur at 

 these mutable loci. The different 

 doses oi Ac together with the changed 

 states of Ac control the time of occur- 

 rence of these mutations. The changes 

 in time of occurrence of visible mu- 

 tations are thus reflections of changes 

 in dosage or changes in state of Ac. 



The mutable loci that require no ac- 

 tivator show the same kinds of expres- 

 sion of variegation as do the activator- 

 requiring mutable loci. It has been 

 shown that the changes occurring at 

 Ac are much the same as those occur- 

 ring at Ds. Thus, Ac or Ac-Yike. loci, 

 could be responsible for the origin of 

 new mutable loci when transposed to 

 a position adjacent to a gene whose 

 inhibited action could be detected by a 

 visible change in phenotype. Dosage 

 action could be exhibited by such 

 autonomous mutable loci, as well as 

 various "changes in state," reflected 

 by changes in the phenotype expres- 

 sion and the time and frequency of 

 occurrence of visible mutations of the 

 affected genes. The study of the be- 

 havior of Ds in its several states makes 

 it possible to reinterpret the variega- 

 tion patterns in Drosophila, which in 

 some cases appear to be associated with 

 loss of segments of chromosomes and 

 in other cases appear to be associated 

 with changes in the degree of action 

 of the genes involved. It also makes it 

 possible to interpret the reported "po- 

 sition-effect" in Oenothera, because 

 the events responsible for the changes 

 in phenotype and the appearance of 

 duplications and deficiencies in this or- 

 ganism appear to be the same or simi- 

 lar to those described for Ds in maize.'^ 



The possible composition of Ac may 



•"> Catcheside, D. G., /. Genet. 38:345-352, 

 1939; Ibid. 48:31-42, 1947; Ibid. 48:99-110, 

 1947. 



