The Genetic Control of Mutation 



385 



is white; if it dissociates during kernel for- 

 mation, the kernel shows colored sectors or 

 dots on a white background; if Ds is moved 

 before the kernel forms, the kernel and later 

 generations of plants arc completely colored. 

 In some plants the colored specks are large, 

 due to the movement of Ds early in develop- 

 ment; in other plants they are small due to the 

 movement of Ds later in development, when 

 very few additional cell divisions take place. 

 The mutation involved here is the loss or re- 

 moval of Ds via breakage. The change in 

 color apparently is not a mutational but a 

 phenotypic event which enables the detection 

 and proof of the mutational event. This 

 phenotypic effect is dependent upon the rela- 

 tive position of genes; that is, it is a position 

 effect; the presence of Ds next to the gene 

 for color suppresses color formation; its ab- 

 sence permits the gene for color to produce 

 color. 



Although the breaks which Ds causes are 

 usually near Ds in the chromosome, they are 

 not always at the same locus. For this rea- 

 son and also 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 after breakage but may transfer 

 from one chromosomal position to another 

 in the same or a different chromosome. As 

 the result of the movement of Ds to new 

 positions, the number of Ds factors present 

 in the endosperm can increase in successive 

 generations. When the number of Ds genes 

 in a given region of a chromosome increases, 

 the region breaks more and more frequently. 



Ds transposed to another chromosome 

 can cause breaks near its new location. 

 Thus, whenever Ds moves, a mutation has 

 occurred. Such relocations of Ds often sup- 

 press the phenotypic effect of a gene located 

 near the new locus of Ds. As long as Ds 

 remains in its new position, the new pheno- 

 type is produced, thereby simulating a stable 

 point mutation of the gene near Ds. More- 



over, each time Ds is lost from such a loca- 

 tion, the new phenotype of the adjacent gene 

 reverts to the old phenotype. If these trans- 

 positions are frequent, they may be incor- 

 rectly scored as point mutations of an un- 

 stable, mutable allele of the neighboring gene. 

 If Ds rarely moves, one might incorrectly 

 score the new phenotype produced by its 

 neighboring gene as a rare mutational change 

 in the neighboring gene. Although it is still 

 not definite how many events scored as point 

 mutations of a given gene are position effects 

 due to suppression or release resulting from 

 a change in linear gene neighbors, not all 

 point changes can be such position effects, of 

 course, since differences among the genes 

 involved in position effects must first arise 

 by mutation. 



The ability of Ds to cause breaks in chro- 

 mosomes is controlled by Activator, Ac, 

 genes. Ac does not have to be on the same 

 chromosome as Ds and usually is not; it also 

 seems to be located in heterochromatin. By 

 appropriate crosses kernels can be obtained 

 whose endosperm contains none, one, two, 

 or three Ac genes in addition to one Ds gene 

 located near a pigment-producing gene (Fig- 

 ure 30-1 ). In the absence of Ac, no specks 

 are produced, and the kernel is completely 

 white. This observation proves that Ds can- 

 not cause chromosome breakage (and is not 

 relocated in other ways) in the absence of 

 Ac. Moreover, as the dosage of Ac increases 

 from one to three, the colored spots become 

 smaller and smaller. Thus, Ac also acts to 

 delay the time of Ds action. Here, then, is 

 a case in which the genotype regulates its 

 own mutability — Ac not only determines the 

 ability of Ds to produce breakages but regu- 

 lates the time in development when breakage 

 is to occur. Ac is clearly acting as a regu- 

 lator gene. This kind of gene may be im- 

 portant in cyclical metabolic processes as 

 well as in cellular differentiation and em- 

 bryonic development. Factors like Ac are 

 fairly common in maize, and the phenotypic 



