MCCLINTOCK 



difference may be related to differ- 

 ences in the inserted chromatin is sug- 

 gested by the appearance of a mutable 

 wx locus arising from a IVx locus in a 

 gamete of a plant carrying c-7n2 ( U'a', 

 starch of endosperm stains blue with 

 iodine; wx, recessive allele, starch stains 

 red with iodine; located in short arm of 

 chromosome 9, proximal to C). The 

 type of variegation expressed by this 

 mutable ivx locus (wx-ml) is strik- 

 ingly similar in all respects to that 

 occurring at c-in2. It could not be de- 

 termined in this case that transposition 

 to the Wx locus of the same inhibiting 

 substance that induced c-7}?2 had oc- 

 curred. Such an event is suspected 

 from the known transposition ca- 

 pacities of this material. 



In this report, Ds, c-ml, c-m2 and 

 ixix-vil have been used as illustrations 

 of newly arising mutable loci because 

 all of them require an activator and 

 all respond to the same activator. This 

 activator has been designated Ac. Ex- 

 tensive studies of Ac have shown that 

 it is inherited as a single unit. It shows, 

 however, a very important character- 

 istic not exhibited in studies of the 

 inheritance of the usual genetic fac- 

 tors. This characteristic is the same as 

 that shown by Ds. Transposition oi Ac 

 takes place from one position in the 

 chromosomal complement to another— 

 very often from one chromosome to 

 another. Again, as in D^, changes in 

 state may occur at the Ac locus. These 

 changes in state are of two main types: 

 either changes that resemble the 

 known effects produced by different 

 doses of the Ac locus from which it 

 was derived, or changes that result in 

 a decidedly altered time of action and 

 dosage response of Ac. Ac may be 

 detected and its action studied by ob- 

 serving the mutations occuring at the 

 mutable loci requiring its presence for 

 mutability to be expressed. It should 

 be emphasized that when no Ac is 



205 



present in a nucleus, no mutation-pro- 

 ducing events occur at c-ml, c-ni2 or 

 "tcx-fnl; nor are any chromosome 

 breakas^e events detected at Ds, for no 

 such events occur. As an example of 

 this interaction it may be stated that 

 c-uil has been maintained in cultures 

 having no Ac locus for several genera- 

 tions, and has given completely color- 

 less aleurone with no evidence of c to 

 C mutations. Similarly, the various 

 quantitative alleles arising from mu- 

 tations of c-7n2 or wx-7}il may be 

 maintained without giving mutations, 

 if Ac is removed from the nucleus by 

 appropriate crosses. Thus a series of 

 stable recessive mutations or stable 

 alleles of a mutable locus may be iso- 

 lated and maintained (if the chromo- 

 some complement is normal, see be- 

 low). When Ac is returned to the nu- 

 cleus, however, instability may again 

 appear. 



The dosage action of Ac may be 

 studied in the diploid plant or in the 

 triploid endosperm tissue of the ker- 

 nels. When marked dosage effects are 

 produced by a particular state of Ac, 

 they are registered alike in both the 

 plant and the endosperm tissues; the 

 higher the dose of Ac, the more de- 

 layed is the time of occurrence of mu- 

 tations at the /ic-controlled mutable 

 loci. Ac determines, therefore, not 

 only the mutation process at these mu- 

 table loci but also the time at which the 

 mutations occur, the different states of 

 Ac giving different times of occur- 

 rence in 1, 2 or 3 doses. The action of 

 Ac on the mutable loci it controls has 

 been described. It is believed that this 

 action produces a stickiness of the in- 

 hibiting materials adjacent to the af- 

 fected loci. With reference to Ds, the 

 observed consequences of this sticki- 

 ness have been enumerated. This 

 physical change probably takes place 

 in the inserted inhibiting materials at 

 all the .(4^-controlled mutable loci at 



