14 



NUTATION AND PLANT BREEDING 



(3o (mutated element) has a null effect which, so far as the aleurone 

 phenotype is concerned, is equivalent to the colorless phenotype 

 associated with recessive a. The validity of this hypothesis may be 

 tested objectively by isolating the active beta element from the com- 

 plex through a crossover event and subjecting it to appropriate 

 mutational analysis to determine whether it has the predicted 

 capacity to mutate to the null level, and if so, whether the frequency 

 of this event corresponds to the frequency of occurrence of the 

 nonrecombinant alpha derivatives from the /3a complex. 



In order to provide a situation that permits the simultaneous 

 comparison of mutation rates of the isolated beta element and the 

 beta element in the beta: alpha complex, and that would eliminate 

 the effect of modifiers, crosses were made to produce marked heterozy- 

 gotes having the constitution T Ba Sh/N B sh, in which the isolated 

 beta element is the same as that in the beta: alpha complex of the 

 homologue. The steps involved in producing the desired heterozy- 

 gotes are shown diagrammatically in Figure 2, which emphasizes the 



T fio «c Sh 



n°^ ' ' ' ' 



N fl <£. Sh T /3 oC Sh J*^^ Nonrecombinant (pale) 

 1 1 p 1 1 1 1 1 nco-l 



nco-2 



sh 



sh '"--^. Nonrecombinant (colorless) 



1 i 1 



N /3 a sh 



A. B. C. 



Figure 2. — Diagrammatic presentation of the steps involved in testing 

 the hypothesis of gene mutatoin of beta to beta () . A, An exchange between 

 beta and alpha isolates beta on A r B sh strand. B, Appropriate crosses 

 are made to produce marked F 1 individuals of the type shown here, 

 carrying the isolated beta in one chromosome and the betaialpha com- 

 plex from which it was isolated in the homologue. C, On the hypothe- 

 sis, gene mutation of beta to beta in the l\ parent should produce type 1 

 nonrecombinant strands (pale phenotype) and type 2 nonrecombinant 

 strands (colorless phenotype), with equal frequencies. 



common origin of the two beta elements in the tested individual and 

 also indicates the strand constitutions of exceptional offspring 

 expected on the hypothesis that mutation of the beta element is 

 responsible for the nonrecombinant alpha derivative in question. If 



