laughnan: nature of mutations 5 



levels, it is obvious that, where technical difficulties place a limit on 

 the size of population that may be analized effectively, our studies of 

 the mutation phenomenon have unwittingly selected against the gene 

 mutational event. We should be prepared to concede then that how- 

 ever important may be the ultimate qualitative genetic change, the 

 rarity of its occurrence in natural and even in experimentally manip- 

 ulated laboratory populations may make it relatively impregnable to 

 attack compared with other types of changes which make a greater 

 immediate contribution to variability. 



I should like to consider here, in some detail, genetic analyses of 

 certain derivatives of the A h complexes in maize which, on the basis 

 of conventional criteria, appear not to be extragenic in origin and 

 which, for this reason, are of particular interest in connection with 

 the problem of gene mutation. 



The various forms of the A\ gene in maize control the synthesis of 

 varying amounts of anthocyanin pigment in the aleurone layer of the 

 endosperm and in certain vegetative tissues. Under appropriate 

 conditions they are also in control of the type of pigment deposited 

 in cells of the pericarp. The phenotypes of aleurone and plant may 

 range from deep purple, as in the presence of A, the type allele, 

 through intermediate levels, to colorless aleurone and brown plant 

 characteristic of individuals that are homozygous for the recessive a 

 allele. In pericarp tissue the type allele A produces a red pigment and 

 is dominant to recessive a, associated with brown pigmentation. The 

 several alleles that go under the general designation A b , and their 

 mutant derivatives, are unique in that they have divergent effects 

 which will not allow their placement in consistent linear array with 

 sister alleles. Thus, A h , which was first described in a stock from 

 Ecuador (5) 2 has a brown pericarp phenotype that is dominant to A, 

 yet is weaker than the latter in its effect on plant pigmentation (6). 



Following the original finding (22) that A b mutates to an inter- 

 mediate allele designated A A (dilute), with a frequency of about 5 X 

 10 4 , detailed analyses (7, 8) employing suitable marker genes have 

 established that A h consists of two closely linked, but separable, ele- 

 ments, both concerned with anthocyanin pigmentation. 



This conclusion is based on an analysis of marker constitutions 

 of A d strands derived from A h ja heterozygotes that carried various 



2 See References, page 25. 



