GENETIC BASIS 37 



bound to occur, the greater will be the recombination. It 

 will readily be seen, however, that, even if we take those or- 

 ganisms with the largest numbers of chromosomes, the most 

 chiasmata, and the least localized chiasmata, we are still 

 much closer to the hypothetical extreme of complete linkage 

 than we are to the other extreme of no linkage. Even under 

 the least effective conditions, the fact that the genes are 

 situated in long, protein structures has a powerful effect upon 

 specific and racial cohesion. 



Linkage has two restrictive effects upon recombination. 

 It limits the numbers of types of different recombinations 

 that can be achieved in any one generation, irrespective of 

 population size. It also affects the frequency with which any 

 particular recombination type can occur. Recombinations 

 requiring a linkage break will, of course, appear with reduced 

 frequencies. In dealing with multiple-factor characters 

 where very large numbers of genes are concerned, the fre- 

 quency of practically every recombination is affected. The 

 effect of linkage upon frequencies had been apparent to many 

 geneticists and was specifically discussed by D. F. Jones in 

 1920. ''Two factors in each chromosome so spaced as to have 

 10 per cent breaks in the linkage with each other would neces- 

 sitate 20^° individuals in the segregating generation to have 

 an even chance of securing the one plant desired. This num- 

 ber of corn plants would require an area roughly 3,700,000, 

 000,000 times the area of the United States.'' (Jones, 1920). 



The restriction imposed even upon populations infinite in 

 size was first pointed out by Anderson in 1939. The follow- 

 ing discussion has been slightly condensed from the original 

 accounts (Anderson, 1939a and b) : 



The restraint of linkages imposes severe restrictions upon 

 the kinds of gene combinations that are possible wdth any 

 frequency. When all the loci of a germplasm are considered, 

 this restriction is as important as that imposed upon fre- 

 quencies and runs into figures of astronomical magnitude. 

 Some notion of its greatness may be gained by considering 

 recombination in a single crossover segment of the germ- 



