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 
would 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 between those loci. If 
then the Mendelian factors lie 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, 7.e., an experiment 
in which three pairs of factors are involved. 
