34 GENE RECOMBINATION 



species to the next. Such variation is almost certain to be con- 

 tinuous in such a way that hard and fast categories could not con- 

 ceivably be applied. Thus the attempt which follows is designed 

 merely to give a very rough outline for this formulation of the 

 problem. The point should again be stressed that it is in almost 

 every case necessary to speak in terms of potential rather than 

 actual recombination. Obviously, too, recombination can be effec- 

 tive only where there are differences which can be recombined. 

 Precise methods do not as yet exist for estimating the total 

 amount of recombinable heterozygosity in a species or a popula- 

 tion. Heterozygosity has its ultimate origin in the mutation proc- 

 ess, but its existence in any given population will depend on 

 many different factors such as possible recent wide outcrossing, 

 including species hybridization, a recent or past high rate of 

 mutation, or recent drastic fluctuations in population size. 



A recombination system wherein a high degree of recombina- 

 tion is permitted may be referred to as open. The human species, 

 for example, displays all the ingredients which may be associated 

 with such a system. The chromosome number (2n = 48) is high, 

 so that a substantial amount of recombination occurs by the seg- 

 regation of whole chromosomes alone, quite apart from crossing 

 over. The number of chromosomally different gametes, which can 

 potentially be formed by an individual which carries one pair of 

 allelic differences per pair of homologous chromosomes is given 

 by the simple formula 2", where n is the number of chromosome 

 pairs concerned. In the present example, the number is 2 24 , more 

 than 16 million, produced by a single individual. Perhaps even 

 more important, however, there is evidence that crossing over 

 between homologues is extensive and unhampered throughout 

 the genome. This system is superimposed on the foregoing one. 

 If the assumption is made that a moderate number of allelic 

 differences exist per chromosome pair, a fantastically high re- 

 combination potential is provided on an individual basis. 



Even under rather close local inbreeding systems, the purely 

 chromosomal devices mentioned above have high potential 

 effectiveness. But superimposed on these internal mechanics are 

 the intricate and far-reaching influences of the choice of mates, 



