The Genetic Control of Mutability 



217 



pw pw vvhile the medium variegated indi- 

 viduals tiiat produced some full red kernels 

 were only heterozygous for /"^', the other 

 allele, P^ , being an unstable one which fre- 

 quently mutated in somatic cells to a red- 

 producing alternative. According to this 

 view, large red sectors are due to mutations 

 of this unstable allele early in development 

 of the shoot, and small sectors are due to 

 mutations which occur later. 



It was found, however, that medium varie- 

 gateds produce not only red mutants but light 

 variegated ones, also. The parental type 

 and the two mutant types can be seen in the 

 ears shown in Figure 25-3. The light varie- 

 gated kernels (''lights^') have about half as 

 many sectors mutant as have medium varie- 

 gated kernels (''mediums""). 



The results of test crossing mediums 

 (^pv pw^ gj-g shown in Figure 25-4. As ex- 

 pected, half of the offspring are nonred 

 (^pw pwy Of the remaining half, which have 

 various degrees of red, 90% are mediums 

 (pF pw^^ about 6% are full red {''reds""), and 



LIGHT 

 (Mutant) 



MEDIUM 

 (Parental) 



FULL-RED 

 (Mutant) 



FIGURE 25-3. Corn ears showing medium vari- 

 egated pericarp (parental type) (A), and the mu- 

 tant types light variegated (B) and full red (C). 

 (Cour testy of R. A. Brink; reprinted by permis- 

 sion of McGraw-Hill Book Co., Inc., from Study 

 Guide and Workbook for Genetics by I. H. Hers- 

 kowitz. Copyright, 1960.) 



4% are lights. The similar frequency of reds 

 and lights suggests that these two mutants are 

 related in some way in their origin. Reds 

 X /^"' P"' produce offspring which, if colored, 

 are all red, the red allele being a stable one. 

 While this result is readily understandable, 

 the following one is not. Lights X P"' P"' 

 sometimes produce about equal numbers of 



FIGURE 25-4. Results of test crossing medium variegated pericarp with colorless pericarp. 



INBRED INBRED 



MEDIUM VARIEGATED COLORLESS 



PERICARP PERICARP 



50 

 Colore 



i{ 



Medium Variegated 90% 

 r Red 6% 



Mutant 



[Light 4y^ 



Variegated 



50% 



Colorless 



(Discarded) 



