34 



INTROGRESSIVE HYBRIDIZATION 



variety of recombinations, they can all be summarized as a 

 general trend from recombinations more or less like one of 

 the parental species, through those much like the Fi, to those 

 more or less like the other parental species. In the following 

 chapter, in considering the effects in later generations, we 



Fig. 4. The ''recombination spindle" of Nicotiana Langsdorffii X 

 N. alata. The theoretical recombinations, A, B, C, D, E, and F, of Fig. 2 

 would be at six corners of the cube of expectations. Tube length is 

 measured on one axis, limb width on another, and lobing on the third. 

 The recombinations form a spindle extending diagonally across the cube. 

 On its surface are the actual extreme recombinations (D', E', etc.), which 



are diagrammed to scale in Fig. 3. 



shall have occasion to refer repeatedly to this ''recombina- 

 tion spindle." 



A theoretical consideration of what we might expect in 

 hybrid populations brings us to exactly the same conclusions 

 as did the experimental evidence from Nicotiana and the 

 practical experience of plant breeders: There are strong co- 

 hesive forces within the germplasm. Although the germ- 

 plasm may well be made up of unit genes (as most geneticists 



