CHAPTER X 

 INTERFERENCE 



One of the most significant results that a study of 

 crossing over has brought to light is that whole blocks 

 of genes go over together. Thus, if one series he A B C D 

 EFGHIJKLMN and its allelomorphic series he ab c 

 d e f g li i j hi m n crossing over may give two blocks of 

 genes. 



ABCDEfghijh I m n 

 a h c deFGHIJKLMN 



This result can best be demonstrated in cases where a 

 number of loci are followed at once. 



The fact that crossing over takes place in blocks is 

 highly significant for the phenomenon of distribution, 

 since it means that pairs of linked genes do not act inde- 

 pendently of their neighbors. This fundamental relation 

 was not suspected until quite recently. 



The size of the blocks, when only one crossing over 

 occurs between the chromosome pairs, depends on the loca- 

 tion in the series of the breaking point. If the crossing 

 over occurs near the middle, the four pieces will be of the 

 same length as shown below: 



If it is near the end of the series, two of the resulting 

 pieces will be small, the other two large. Thus : 



a h c DEFGHIJKLMN 



ABC d e f g h i j k I i)h n 



The two ''like'' pieces in all cases contain identical series 

 of loci. 



126 



