166 



CHAPTER 37 



? A 



LIGHT 



(Mutant) 



MEDIUM 

 (Parental) 



FULL-RED 



(Mutant) 



figurf. 37-2. Corn ears showing medium 

 variegated pericarp (parental type) {A), and 

 the mutant types light variegated (B) and full 

 red (C). {Courtesy of R. A. Brink; reprinted 

 by permission of McGraw-Hill Book Co., Inc., 

 Horn Study Guide and Workbook for Genetics 

 by I. H. Herskowitz. Copyright, I960.) 



(/' P ■ ). In the remaining half, of those 

 with various degrees of red. 90% are me- 

 diums {P' P"); about 6% are ///// red 

 (reds); and 47c are lights. The similar 

 frequency of reds and lights indicates that 

 these two mutants may somehow be related 

 in origin. Reds by P" P" produce offspring 

 which are all red. if colored at all. The red 

 allele is stable in this cross. 



Occasionally, medium ears show the two 

 mutants, light and red. as twin patches of 

 kernels (Figure 37-4). This situation sug- 

 gests that reds and lights are not merely re- 

 lated to one another in origin but that they 

 are complementary. In other words, one 

 has gained something the other has lost in 

 the mutation process. In view of the results 

 with Ac and Ds, a new genetic hypothesis 

 can be presented to explain pericarp variega- 

 tion (Figure 37-5). Note that the gene 

 symbols are changed. The way stocks of 

 these strains are maintained, all variegated 



genotypes are heterozygous for P" , the stable 

 gene (on chromosome 1 ) for nonred peri- 

 carp. The variegated allele is considered a 

 dual structure, containing P' , the top domi- 

 nant allele for red and Mp, Modulator, which 

 suppresses red pigment production. 



Since a P' Mp combination suppresses red 

 pigmentation, mediums are produced. P' 

 alone produces stable, full red; P' Mp. plus 

 an additional Mp somewhere else {trans- 

 posed Modulator), produces lights. Con- 

 sider the results obtained when certain lights 

 {P r Mp P" plus transposed Mp) are test 

 crossed by P" P" . Half the offspring are 

 nonred {P" P"); the other half are colored 

 — about half of them lights (genetically sim- 

 ilar to the light parent), half mediums (sim- 

 ilar to the light parent but lacking the trans- 

 posed Mp) with a few reds (cases where 

 Mp is transposed from P' Mp, leaving P' 

 alone). The mechanism for transposing Mp 

 away from P' Mp is considered the same as 



INBRED 



MEDIUM VARIEGATED 



PERICARP 



50% 

 Colored 



Medium Variegated 90% 



Red 6% 



Mutant 



Light 4% 



Variegated 



INBRED 



COLORLESS 



PERICARP 



50% 



Colorless 



(Discarded) 



figure 37-3. Results of test 

 crossing medium variegated 

 pericarp with colorless peri- 

 carp. 



