GENETIC BASIS 31 



then summarize briefly the special forces that operate in some 

 species crosses but not in others. 



Before considering the theoretical basis of character re- 

 combination in the F2, let us review the facts on the subject. 

 It has already been mentioned that, except in certain ex- 

 ceptional cases, the Fi of a cross between well-marked va- 

 rieties, or between species, is highly uniform, whereas the 

 F2 is extremely variable. These tw^ contrasting generations, 

 the one so outstandingly uniform, the other so outstandingly 

 variable, have caught the imaginations of nearly all those 

 who have worked with them. The hybridizers have been 

 so intrigued by this contrast that they have made little or 

 no effort to catalogue and analyze the variation in F2 popu- 

 lations. There does not seem to be a single published paper 

 in which any attempt was made to determine whether the 

 recombinations of the F2 were infinite in their variety or oc- 

 curred by the scores, by the hundreds, or by the thousands. 

 From most of the descriptions in published papers one would 

 gather that the number of recombinations were infinite; a 

 little research in the tables accompanying these papers will 

 show that a few hundred individuals, at the most, wxre under 

 consideration. Yet it is quite simple to demonstrate (An- 

 derson, unpubhshed) that in any such cross the numbers of 

 recombinations are distinctly finite. It is possible to deter- 

 mine for any particular cross the numbers of F2 individuals 

 that must be grown before one has a good chance of obtain- 

 ing two individuals essentially similar. 



In one published case (Anderson, 19396) a pioneer attempt 

 was made to compare the recombination of the F2 with the 

 recombinations that might have been expected if there had 

 been no restrictions of any sort upon complete recombina- 

 tion. ''In Nicotiana alata X N. Langsdorffii, if we consider 

 only the differences in tube length, in the lobing index, in 

 style length, and in limb width, the recombinations obtained 

 are only %4 of the kinds of recombinations which might be 

 obtained with free assortment. These four characters, how- 

 ever, represent only a few of many differences which might 



