46 MOLECULES, VIRUSES, AND BACTERIA 



arise by two processes: reverse mutation of one of the alleles and re- 

 combination between alleles. Appropriate controls show, in Calef's 

 case, that spontaneous mutation is of the order of 10". However, for 

 each million spores, three are able to grow on adenine-free medium. 

 These must, therefore, arise by recombination, since mutation is 3,000 

 times less frequent. When the genetic constitutions of the chromosomes 

 bearing the adenine locus are examined, one obtains the recombinant 



proline adenine paba 



CROSS A 



+ ^^17 + 

 parent 1 ' 1 1 ; j 



- + ad,c + 

 parent 2 i i i ^^ i 



CROSS B 



parent 1 | i l±2- 



parent 2 



adi7+ H- 



Unselected recombinant types in 406 unselected spores 



+ 



o 



Selected recombinant types in 1.3- 10 spores 



class 1 I I 1 \- 



-\- + + 



4- + + -h 



H h- 



Figure 2. Recombination between adenine alleles in Aspergillus (Calef, 

 1957). Crossing-over within the adenine locus is so rare that it is detected 

 only by selection of adenine spores. The reciprocal recombinant, adi^-adi^, 

 which presumably occurs, cannot be recovered by selection. 



