RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 



71 



cross-over class, it is clear that the origin of the chromosome can be 

 determined only if we know from which end duplication begins, and 

 have a marker near that end. The same is true for the triple cross-over 

 class ( or indeed for any odd number of switches ) . On the other hand, 



CROSS A 



parent 1 

 parent 2 



outside markers 

 1. parent 1 



+ 



ad 



17 4- 



paba 



pro 



-h ad 



15 



+ 



per cent 



of all ad + 



recombinants 



12 



2. parent 2 



26 



3. recomb. 



4. recomb. 



57 



CROSS B 

 parent 1 

 parent 2 



+ ad 



15 



paba 



pro 



adi7 + 



+ 



1. parent 1 



26 



2. parent 2 



11 



3. recomb. 



59 



4. recomb. 



4 



Figure 10. Diagram of the formation of wild-type recombinants in crosses 

 involving two adenine alleles in Aspergillus (Calef, 1957). Recombinants are 

 grouped in classes according to whether the outside markers are in parental 

 arrangements or recombined. These are the four classes of selected re- 

 combinants shown in Figure 2. 



