LINKAGE 61 



they cross, and if they do not then when they begin 

 to separate they simply pull apart irrespective of the 

 side on which they lie. When this occurs each 

 chromosome remains intact and no crossing over 

 takes place. 



Later some of the evidence on which the above 

 statements rest will be examined more critically. 

 For the present it need only be pointed out that 

 such a crossing over of parts of the chromosomes 

 w^ould supply the necessary mechanism to account 

 for interchange. If the crossing over may occur at 

 any point in a chromosome, then the chance of its 

 occurrence between two given loci will be greater, 

 the greater the distance betw^een those loci. If 

 then the Mendelian factors He along the chromo- 

 somes, the amount of crossing over between any two 

 of them will depend on their distance apart. Should 

 two points lie near together a crossover will only 

 rarely occur between them; if they lie further apart 

 the chance of such a crossover taking place at some 

 point between them will be greater. From this 

 point of view the percentage of crossing over is an 

 expression of the "distance" of the factors from each 

 other. 



In this way the diagram shown in the frontispiece 

 has been constructed. Not only can all the facts 

 of linkage so far studied be explained on this basis, 

 but, as will now be shown, certain further results can 

 be predicted. This is illustrated in what may be 

 called a three-point experiment, i.e., an experiment 

 in which three pairs of factors are involved. 



