THE MENDELIAN PRINCIPLES OF HEREDITY 95 



seen to twist around each other (Fig. 50) - - is not 

 certain ; but the fact of the existence of cross connec- 

 tions is the important point. A consequence of this 

 condition is that the chromosomes that come out of 

 the tetrad may represent different combinations of 

 those that united to form the group. On the basis 

 of this observation we can explain the results of associ- 

 ated inheritance. For, to the same extent to which 

 the chromosomes that unite remain intact, the factors 

 are linked, and to the extent to which crossings occur 

 the exchange of factors takes place. On the basis 

 of the assumption of the linear arrangement of the 

 factors in the chromosomes the distance apart of the 

 factors is a matter of importance. If two factors 

 lie near together, the chance of a break occurring be- 

 tween them is small in proportion to their nearness. 

 We have found that some factors cross over not once 

 in a hundred times. I interpret this to mean that they 

 lie very near together in the chromosome. 



Other factors cross over to various degrees ; in the 

 extreme cases the chance is one to one that they cross 

 over. In such cases the factors lie far apart — perhaps 

 near the ends of the chromosome. 



The strongest evidence in favor of this view is found 

 when the constant relation of the factors to each other 

 is considered. If, for instance, we know the distance 

 from A to B (calculated on the basis of crossing over) 

 and from B to C, we can predict what A and C will do 

 when they are brought into the hybrid from two 

 parents. If a fourth factor, D, is discovered and its 

 distance from A is made out, we can predict before the 

 experiment is made what will take place when D and 



