Regulation of Gene Action — Gene Control Systems in Maize 



467 



AJbO V 



figure 37-4. Twin patch of mutant 

 kernels, full red and light variegated, 

 in a medium variegated pericarp ear. 

 (Courtesy of R. A. Brink; photograph 

 LIGHT by The Calvin Company reprinted by 



VARIEGATED permission of McGraw-Hill Book Co., 

 Inc., from Study Guide and Workbook 

 tor Genetics by I. H. Herskowitz. Copy- 

 right, I960.) 



that for transposing Ds. This situation is 

 illustrated in Figure 37-6, where the medium 

 parent cell chromosomes (P ' Mp and P" ) 

 are shown already divided, but daughter 

 strands are still connected at the centromere. 

 Normal division would have produced two 

 daughter cells — each carrying P' Mp P" , 

 each giving rise to medium sectors. But 

 when transposition occurs as a result of two 

 or more breaks or some other mutational 

 mechanism, and, consequently, the Mp in 

 one daughter strand is transposed into a non- 

 homologous chromosome (hollow bar), then 

 the daughter cell which receives the trans- 

 posed Mp may be the one carrying P r Mp, 

 while the other daughter cell will carry only 

 P' . Thereafter, normal mitosis of the cell 

 containing P' alone will produce reds; and 

 mitosis of the sister cell will produce lights. 

 These cells will then become adjacent mu- 

 tant patches in a medium background (Fig- 

 ure 37-4). 



Red by nonred (P r /P u X P"/P") should 



produce roughly one-half nonred and one- 

 half red. As mentioned previously, it does. 

 Reds do not have Mp adjacent to P'\ Light 

 by nonred (> Mp P" plus transposed 

 Mp x P" P") always produces half nonred. 

 When transposed Mp is located in a non- 

 homologous chromosome, about one quarter 

 of Fj will be light and one quarter will be 

 medium, as mentioned in the previous para- 

 graph. Mp can move away from WWp 

 yet remain in the same chromosome at a 

 new position, so lights may still have their 

 transposed Mp on chromosome 1. In this 

 instance, backcrossing will produce more 

 than one quarter lights in F, and correspond- 

 ingly fewer mediums. 



Other properties of Mp have been dis- 

 covered. Mp may become fixed at the P 

 locus so that a medium becomes a stable 

 nonred form. Transposed Mp may occupy 

 a variety of sites; two of these have already 

 been mentioned (linked and no-longer-linked 

 to chromosome 1 ). In 57 out of 87 cases, 

 transposed Mp was found still linked to 



PHENOTYPE 



GENOTYPE 



Medium 

 Variegated 



P Mp / P 



Mutants 



Red 



Light 

 Variegated 



r w 



P /P 



P Mp / P + Transposed Mp / - 



figure 37-5. New 

 genetic hypothesis 

 for pericarp variega- 

 tion. 



