laughnan: nature of mutations 25 



and from bar homozygotes carrying two normal X chromosomes. In 

 addition, double bar has given rise to nonrecombinant bar offspring, 

 and bar in turn, has produced noncrossover double-bar types. These 

 derivatives are consistent with expectations on the A.A. hypothesis 

 though their occurrence is not of itself decisive. It is of interest to 

 note that Sturtevant, in his classical work on bar (24), recorded several 

 cases of changes at the locus which were not associated with crossing 

 over, though he considered their status as valid cases somewhat doubt 

 ful. Also, Braver (2), in the course of an experiment designed to test 

 the effect of a nearby inversion on unequal crossing over at the bar 

 locus, has obtained (personal communication) a nonrecombinant 

 wild-type revertant from a homozygous-bar parent. 



Thus, it appears that the bar duplication has its share of anoma- 

 lous derivatives, and cytological analysis of those now in hand may 

 be expected to shed light on a phenomenon which, though it is not 

 attributable to gene mutation, is certainly a subtle substitute. On the 

 basis of available evidence it may be anticipated that the phenomenon 

 dealt with here is significant not only for the contribution it makes 

 to variability through regressive changes, but for the progressive 

 evolution of gene systems as well. 



References 



1. Anderson, E. G., and Brink, R. A. 1940. Translocations in maize 



involving chromosome 3. Genetics, 25: 299-309. 



2. Braver, G. 1960. The influence of an adjacent inversion breakpoint 



on unequal crossing over in the Bar region of Drosophila mel- 

 anogaster. Records Genetics Soc. Amer., 29: 59. 



3. Emmerling, M. H. 1956. Unequal crossing over between elements 



of the R complex in Zea mays. Genetics, 41: 641. 



4. . 1958. An anaylsis of intragenic and extragenic mutations 



of the plant color component of the R r gene in Zea mays. Cold 

 Spr. Harb. Symp. Quant. Biol, 23: 393-407. 



5. Emerson, R. A., and Anderson, E. G. 1932. The A series of allelo- 



morphs in relation to pigmentation in maize. Genetics, 17: 503- 

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6. Laughnan, J. R. 1948. The action of allelic forms of the gene A in 



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7. . 1949. The action of allelic forms of the gene A in maize: 



