laughnan: nature of mutations 13 



Table 4. — Constitutions of Alpha-bearing Strands from .4 b -P/Df a-X Hemizygotes. * 



A h -P Distribution of alpha-bearing 



Source gimetes strands among offspring 



tested 



T /l b -Lima Sh/N a-Xl 1 32,640 93 T a Sh 



T /l b -Cusco Sh/N a-Xl 400 1 TaSh 



T .4 b -Lima Sh/N a-X3 20,580 12 T a Sh 



T A h -Cus,co sh/N a-X3 2.350 3 T a sh 



No marking: 



N ^ b -Lima Sh/N a-Xl 1 28.280 83 N a Sh 



N ^ b -Cusco Sh/N a-Xl 86.355 44 N a Sh 



N A h -Uma Sh/N a-X3 4.555 4 N a Sh 



N ^ b -Cusco Sh/N a-X3 2,500 1 . V a Sh 



*The deficient segments in both Df a-Xl and Df a-X3 include the A locus and the Sh locus. 

 See text for further details. 



deficiency may extend well to the left of the A locus. In any case, the 

 regular occurrence of alpha strands from these deficiency heterozy- 

 gotes indicates clearly that the phenomenon leading to the nonrecom- 

 binant alpha occurrence can not be attributed to any mechanism 

 requiring the direct participation of the homologue. 



One obvious explanation for the occurrence of the nonrecombi- 

 nant alpha derivatives that does not conflict with the evidence so far 

 presented would attribute them to mutation of the beta element. 

 Since the beta element of the complex has a purple effect, it masks the 

 pale or intermediate phenotype associated with the adjacent alpha 

 component. A qualitative change in the beta element of the complex, 

 rendering it ineffective in the production of pigment, would allow 

 the expression of the adjacent alpha element under circumstances 

 which would not involve the participation of the homologue and 

 which would thus be independent of recombination for marker loci. 

 However, since the crossover alpha derivatives (co-1) which have 

 lost the beta element in the exchange event are phenotypically indis- 

 tinguishable from the nonrecombinant alpha individuals (nco-1), 

 the restriction is imposed that the presumed mutation of the beta 

 element be to the null level, hereafter referred to as beta or /? - 



The gene mutation hypothesis proposed here equates the occur- 

 rence of the nonrecombinant alpha with a change in the A h -P com- 

 plex from /? : a to /?,, : a, where the former has the typical purple 

 effect, the latter the typical alpha (pale) effect, and the hypothetical 



