42 INTROGRESSIVE HYBRIDIZATION 



or hundreds of gene differences are concerned in species 

 crosses (as has been assumed by those geneticists who have 

 made serious attempts to obtain data on this difficult point), 

 then it is a force so great as to require scores of generations 

 of controlled breeding before it could be completely nullified. 

 In natural populations the effects of linkage upon gene fre- 

 quencies are equally important, and they will be discussed 

 in the following chapter. 



Were the science of cytogenetics further advanced it 

 might be instructive to calculate the cohesive effect of link- 

 age in a set of limiting cases. We are still at the point, how- 

 ever, where we have to make too many assumptions in lieu 

 of actual data. We do not have any exact information (even 

 exact estimates) as to the number of gene differences between 

 species. As important as data on gene number are data on 

 chiasma frequency and localization. Chiasmata are the re- 

 sult of exchange between homologous chromosomes at the 

 reduction division. The greater the chiasma frequency, the 

 larger is the number of units in which the germplasm may 

 be shuffled, and the less is the cohesive effect. Quite as im- 

 portant for our purpose are data on chiasma localization. 

 From cytological observation we know in a rough way that 

 in some species chiasmata are highly localized; that is, they 

 tend very strongly to occur in certain parts of the chromo- 

 somes. In other species no such tendency is clearly mani- 

 fest, and they are said (by cytologists) to occur at random. 

 For a precise computation of the cohesive effect of linkage 

 we need to know just how randomized the chiasmata are. 

 The more they tend to be localized, the less variation there 

 will be in gene combinations between sister germ cells and 

 the stronger will be the cohesive force of linkage. Chiasma 

 munber determines the number of segregating blocks in the 

 germplasm. Localization determines how closely the blocks 

 produced by any one pollen mother cell of a plant resemble 

 those produced by its sister cells. 



Among the higher plants the available data would suggest 

 that an average condition might be something like 12 pairs 



