Chapter *37 



REGULATION OF GENE 

 ACTION-GENE CONTROL 

 SYSTEMS IN MAIZE 



I 



"n discussing the genetic control 

 of mutation (Chapter 30), we 

 considered the case of Activator 

 and Dissociation in maize (pp. 384-386). 

 The control of chromosomal breakage in the 

 Ac-Ds system was found to be associated 

 with the way this system controls the func- 

 tioning of genes located in cis position near 

 Ds — Ds being controlled by Ac, a regulator 

 gene. Based on our knowledge of regulator 

 gene-operator gene systems in bacteria, we 

 can postulate that Ds functions as an opera- 

 tor gene for the Ac regulator gene. 



Let us analyze another case ' of what 

 originally appeared to be an unstable gene 

 in maize. The pericarp of a corn kernel 

 encloses the seed containing the embryo 

 (Figure 2-8, p. 25). Although embryo 

 tissue is formed by the offspring generation, 

 the pericarp is formed by the parental gen- 

 eration. Some plants are completely red 

 and produce completely red pericarps; other 

 plants are striped with red, and striping ap- 

 pears also in the pericarp; still others are 

 completely nonred. A plant which shows 

 medium variegation of red (therefore called 

 medium variegated) produces kernels of the 

 type shown in Figure 37-1. In the random 

 sample of kernels shown, about 6% have 

 full red color. From this result (and others ) 

 it appears that the parent of a medium 

 variegated pericarp has about 6% mutant 



1 Based upon work of R. A. Brink and co-workers. 

 465 



kernels. Genetically, these results can be 

 attributed to mutation at a locus P on chro- 

 mosome 1, and we can expect nonred indi- 

 viduals to be P" P", and medium variegated 

 individuals producing some full red kernels 

 to be heterozygous for P" . The other allele, 

 P'\ would be unstable and in somatic cells 

 frequently mutate to a red-producing gene. 

 If we accept this hypothesis, large red sectors 

 of the stem and leaves would be due to mu- 

 tations of this unstable allele which occur 

 early in development of the shoot; small 

 sectors would be due to later mutations. 



It has been found, however, that medium 

 variegated individuals produce not only red 

 but also light variegated mutants. The pa- 

 rental type and the two mutant types of ears 

 can be seen in Figure 37-2. The light 

 variegated kernels (lights) have about half 

 as many sectors mutant as have the medium 

 variegated kernels (mediums). 



The results of test crossing mediums 

 (P v P" ) are shown in Figure 37-3. As 

 expected, half the offspring are nonred 



figure 37-1. A random sample of kernels 

 from a medium variegated pericarp ear. 

 (Courtesy of R. A. Brink; photograph by The 

 Calvin Company reprinted by permission of 

 McGraw-Hill Book Co.. Inc.. from Study Guide 

 and Workbook for Genetics by 1. H. Hersko- 

 witz. Copyright, 1960.) 





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