laughnan: nature of mutations 21 



over alpha derivatives among the progeny, there is again no support 

 for the gene-mutation hypothesis. 



On the basis of the evidence presented above, the contention 

 that gene mutation of the beta element is responsible for the occur- 

 rence of nonrecombinant alpha derivatives is indefensible. This 

 should come as no surprise, however, since it fits a pattern that has 

 by now become commonplace. The A alleles are among the most 

 intensively studied in maize and, of the many "mutations" recorded 

 at this locus, the noncrossover alpha derivatives analyzed here are 

 among the select group that had survived previous tests designed to 

 identify extragenic changes. It is difficult to avoid the conclusion 

 that, if the currently favored hypothesis of gene mutation (25, 26) 

 based on the alteration of structure at the molecular level is valid, we 

 should not expect to encounter the phenomenon in maize in experi- 

 ments which at best can deal statistically with events at the level of 

 10" 5 . It is increasingly apparent that the classical gene mutation eludes 

 the investigator, not so much because it defies characterization, but 

 because it is of such infrequent occurrence that it is swamped by 

 events that are extragenic. We can only conclude that nature is 

 conservative in its display of gene mutations and that the predomi- 

 nant and immediate contributions to variability in the population 

 are functions of various extragenic events. 



In regard to the case discussed here we may summarize as follows: 



(a) The noncrossover event leading to the expression of alpha 

 involves a loss of the beta (purple) phenotypic expression. 

 This follows since the crossover and noncrossover alpha 

 derivatives are identical in phenotype. 



(b) The noncrossover alpha derivative originates through a 

 physical loss, not gene mutation, of the adjacent beta 

 element. 



(c) The experiments dealing with heterozygous deficiencies 

 indicate that the occurrence of the noncrossover alpha 

 derivative (loss of beta) does not involve the participation 

 of the homologue. 



(d) Loss of the beta element of the complex is conditioned by 

 the presence of the adjacent alpha element since it has 

 been shown that the isolated beta element "mutates" 

 rarely, if at all. 



