204 



CELL HEREDITY 



Cross 



Tetrads showing 

 recombination in 

 both intervals 



a b c 

 a + c 

 + b + 

 + + + 



a 6 + 

 a ■¥ c 

 + b c 

 + + + 



a b c 

 a + + 

 + 6 + 

 + + c 



a 6 + 

 a + + 

 + 6 c 

 + + c 



Interpretation on 

 (a) basis of breakage 

 and reunion 



(b) 



Interpretation on 

 basis of copy 

 choice and sister- 

 strand exchange 



FIGURE 7.10. Genetic consequences of copy-choice recombination plus sister-strand 

 exchange (from Pritchard, 1960, Microbial Genefics, Cambridge, The University 

 Press). 



FIGURE 7.11. Diagrammatic 

 representation of a linear 

 model of a chromosome. The 

 DNA particles (molecules?) 

 are held together by links 

 (protein?) which are assumed 

 to provide great flexibility to 

 the over-all structure, which 

 can be stretched out, becom- 

 ing virtually linear, or folded 

 to form a ribbon. A genetic 

 consequence is the possibility 

 of two different kinds of ex- 

 change, one occurring at the 

 link points, and the other oc- 

 curring within the DNA helix, 

 as for instance, in Figure 7.15 

 (after Freese, 1958, Cold 

 Spring Harbor Symp. Quant. 

 Biol., 23:13). 



