laughnan: nature of mutations 27 



24. Sturtevant, A. H. 1925. The effects of unequal crossing over at the 



Bar locus in Drosophila. Genetics, 10: 117-147. 



25. Watson, J. D., and Crick, F. H. C. 1953. Genetical implications of 



the structure of desoxyribose nucleic acids. Nature, 171: 964. 



26. -. 1953. The structure of DNA. Cold Spy. Harb. Symp. Quant. 

 Biol, 18: 123-131. 



Comments 



Stephens: Strains of corn from Ecuador and Peru have the alpha and 

 beta elements arranged in different serial order. Do you have any simple 

 mechanism which would derive one from the other? 



Lauhgnan: A scheme to derive one from the other was produced in con- 

 nection with the original report (PNAS 41: 78-84) on the changed 

 order of elements in the Peruvian complexes. It is based on the find- 

 ing that adjacent alpha and beta segments represent tandem serial dupli- 

 cations whose members retain homology and may thus be obliquely 

 synapsed in meiosis. Given the opportunity for crossovers within these 

 members, and the occurrence of alpha-carrying chromosomes in the 

 population, there are various ways of deriving the beta: alpha order 

 from the alpha: beta order, and nice versa. For example, with exchanges 

 in successive generations, we may go from alpha:beta to alpha:beta: 

 beta to alpha: beta: alpha to beta: alpha. 



Peterson: Can the physical loss of beta be considered in the light of 

 transposition events (simple drop out) associated with mutable gene 

 phenomena? 



Laughnan: It may be assumed that a Ds-like element resides at the 

 beta locus and conditions an infrequent loss of the latter. Against this 

 is the evidence indicating that the isolated beta element, removed from 

 its association with alpha, is remarkably stable. Moreover, somatic alpha 

 occurrences, expected as the rule on the insertion-transposition hypothe- 

 sis, are all but absent, the vast majority of the alpha cases originating 

 in connection with meiosis. And finally, progeny analysis of the alpha 

 occurrences, a routine procedure in our studies, gives no hint of lowered 

 gametophytic or sporophytic viability which would accompany the 

 grosser aberrations predicted to occur at least occasionally on the trans- 

 position hypothesis. 



