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CHAPTER 25 



It was found that the colored speckles 

 were large in some plants, while in other 

 plants they were small in size. The large 

 speckles were due to the movement of Ds 

 early in development, while the small speck- 

 les meant the movement of Ds later in de- 

 velopment, when very few additional cell 

 divisions had yet to take place. It was possi- 

 ble to obtain cells containing one, two, and 

 three Ac genes, the spots becoming smaller 

 the greater the number of Ac genes present. 

 Thus, Ac also acts to delay the time of Ds 

 action. Here, then, is a case where the 

 genotype regulates its own mutability — Ac 

 not only determines the capacity of Ds to pro- 

 duce breakages but regulates the time when 

 breakage is to occur (Figure 25-1). 



The breaks which Ds causes are usually 

 near to Ds in the chromosome, but are not 

 always at the same locus. Because of this 

 property, and because breaks can occur 

 simultaneously in other chromosomes (due 

 to spontaneous events, or to the presence of 

 other Ds genes located in them), Ds need not 

 be lost following breakage, but can become 

 transported from one position in a chromo- 

 some to another in the same or a different 

 chromosome, and the number of Ds factors 

 present can increase in successive generations. 

 When the number of Ds genes increases in a 

 given region of a chromosome, that region 

 breaks more and more frequently, while a Ds 

 transposed to another chromosome can 

 cause breaks near it in its new location. 

 Whenever Ds moves its location, a mutation 

 has occurred. Such relocations of Ds often 

 cause a suppression of the phenotypic effect 

 of a gene located near the hew locus of Ds. 

 As long as Ds remains in its new position, the 

 new phenotype is produced, and this simu- 

 lates a stable point mutation of the gene 

 near Ds. Moreover, each time Ds is lost from 

 such a location, the new phenotype of the 

 adjacent gene reverts to the old phenotype. 

 If these last transportations are frequent, they 

 may be incorrectly scored as point mutations 



of an unstable, mutable allele of the neigh- 

 boring gene. 



Let us next consider in some detail how 

 another case of what originally appeared to 

 be an unstable gene was analyzed in maize.^ 

 The pericarp of a corn kernel encloses the 

 seed which contains the embryo. While em- 

 bryo tissue is of the offspring generation, 

 the pericarp is formed by the parental gen- 

 eration. Some plants are completely red 

 and produce fully red pericarps in their 

 kernels, others are striped with red, this 

 striping being seen also in the pericarp, and 

 others are completely nonred. A plant 

 which shows medium variegation of red, 

 therefore called medium variegated, produces 

 kernels of the type shown in Figure 25-2. 

 Note in the random sample of kernels shown 

 that about six per cent have full red color. 

 From this result and others it was decided 

 that a medium variegated pericarp parent 

 has about six per cent of kernels that are mu- 

 tant. Genetically, the results were inter- 

 preted as being due to mutation at a locus P 

 on chromosome 1. Nonred individuals were 

 2 Based upon work of R. A. Brink and coworkers. 



FIGURE 25-2. A random sample of kernels from 

 a medium variegated pericarp ear. {Courtesy of 

 R. A. Brink; photograph by The Calvin Com- 

 pany reprinted by permission of McGraw-Hill 

 Book Co., Inc., from Study Guide and Work- 

 book for Genetics by I. H. Herskowitz. Copy- 

 right, 1960.) 



