ARCHITECTURE OF THE GERMPLASM 243 



occur. Syndesis has been repeatedly observed and 

 sometimes two chromosomes are seen even to twist 

 about each other. When separation comes after this 

 embrace the two original chromosomes may simply 

 unwind and so regain their former condition unchanged, 

 or they may break and fuse in such a way that one (A) 

 has a part of the other (B), and the remaining parts 

 show a corresponding fusion, as indicated in Figure 73. 

 This is the chromosomal explanation (Chiasmatype 



FIG. 73. Diagram to show cross-over between two homologous 

 chromosomes. After Muller. 



theory of Janssens) of the cross-over phenomena 

 known to the experimental breeder. 



6. INTERFERENCE 



The varying percentages of cross-overs between dif- 

 ferent pairs of genes led Morgan and his associates to 

 attempt the localization of genes within the chromo- 

 somes. The idea, as suggested by Bridges in 1914, is 

 simply this, that the farther apart two genes are in 

 the chromosome the more likely they are to cross 

 over and to exchange places with their homologous 

 genes during syndesis. 



