218 



CHAPTER 25 



%^X^^J 



LIGHT 

 VARIEGATED 



lights and mediums among the colored off- 

 spring, plus a few reds (which can be con- 

 sidered new mutants from variegated). We 

 shall return to this result shortly. 



Occasionally, medium ears show the two 

 mutants, light and red, as twin patches of 

 kernels (Figure 25-5). This suggests that 

 reds and lights are not merely related in 

 origin, but that these mutants are comple- 

 mentary; that is, in the mutation process, 

 one has gained something the other had lost. 

 In view of the results with Ac and Ds, a new 

 genetic hypothesis can be presented to ex- 

 plain this pericarp variegation (Figure 25-6). 

 Note that the gene symbols have been 

 changed. Because of the way these strains 

 are carried and crossed, all variegated geno- 

 types are heterozygous for /^"', the stable gene 

 on chromosome 1 for nonred pericarp. The 

 variegated allele is considered to be a dual 

 structure, containing P% the top dominant 

 allele for red, and Mp, Modulator, which 

 suppresses red pigment production. 



P" Mp together suppress red pigmentation, 

 so that mediums are produced. /*"■ alone pro- 

 duces stable, full red. /"■ Mp, plus an addi- 

 tional Mp somewhere else (transposed Modu- 

 lator), produces lights. This would explain 

 the results, given in the next to last para- 

 graph, from backcrossing lights (P"" /*"' 

 X P' Mp/P"^ plus transposed Mp). Half of 

 the offspring would be nonred (P"* /•">), the 

 colored being about half lights (genetically 

 like the light parent) and half mediums (like 

 the light parent but lacking the transposed 

 Mp) and a few reds (cases where Mp was 

 transposed from P"" Mp, leaving P'' alone). 

 How can the mechanism for transposing Mp 



FIGURE 25-5. Twin patch of mutant kernels, 

 full red and light variegated, in 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 

 Workbook for Genetics by I. H. Hers- 

 kowitz. Copyright, 1960.) 



away from P' Mp be visualized? The same 

 kind of process that transposes Ds can be 

 considered to apply here, also. This can be 

 illustrated with Figure 25-7, where the 

 medium parent cell has chromosomes 

 (P'" Mp and P"") which are shown already 

 divided, daughter strands still being con- 

 nected at the centromere. A normal divis- 

 ion would produce two daughter cells 

 each carrying P^ Mp/P"", and each would 

 give rise to medium sectors. But when mu- 

 tation occurs, presumably involving two or 

 more breaks, the Mp in one daughter strand 

 may be transposed into a nonhomologous 

 chromosome (hollow bar), and it is possible 

 that the daughter cell which receives the 

 transposed Mp will be the one carrying 

 /''■ Mp, in which case the other daughter 

 cell will carry only /"". Subsequent normal 

 mitosis, of the cell containing P'" alone, will 

 produce reds, and of the sister cell, lights, 

 these cells becoming adjacent mutant patches 

 in a medium background (see again Figure 

 25-5). 



Red X nonred {P'/P^ X P"'/P"') should 

 produce about half nonred and about half 

 red. We have already mentioned that it 

 does. Reds do not have Mp adjacent to P^ 

 Light X nonred {P" Mp/P"" plus transposed 

 Mp X P"" P"") should produce half nonred. 

 It does. If, in the last cross, transposed Mp 

 is located in a nonhomologous chromosome, 

 one quarter of Fi will be light and one 

 quarter will be medium. But Mp can move 

 from P'" Mp, yet still remain in the same chro- 

 mosome but at a new position. Lights may 

 therefore have their transposed Mp on chro- 

 mosome 1. In this case, backcrossing will 



