244 



GENETICS 



Of course if they lie very close together in the chro- 

 mosome they are apt to be found finally on the same side 

 regardless of the twisting of the paternal and maternal 

 chromosomes about each other. This is evident in 



_ Figure 74 where the invisible 



genes are represented hypo- 

 thetically by letters placed 

 within the chromosomes. Cross- 

 over is more likely to occur be- 

 tween A and E which lie at the 

 extremes of chromosome I than 

 between A and B which are 

 closer together. 



Again, when genes lie close 

 together they theoretically in- 

 terfere with the crossing over 

 of neighboring genes as pointed 

 out by Muller and confirmed by 

 subsequent breeding experi- 

 ments. In Figure 74, for ex- 

 ample, if crossing-over took 

 place between the pairs Cc and 

 Dd, breaking the linkage be- 

 tween C and D and between c 

 and d, it would prevent another 



break of linkage between BC and be. This is the phe- 

 nomenon of interference. It follows that the nearer 

 together two pairs of genes involved in cross-over are 

 located, the greater will be the interference. 



FIG. 74. Interference. 

 Two homologous chro- 

 mosomes during syn- 

 desis. When there is 

 a cross-over between 

 Cc and Dd, it inter- 

 feres with another 

 cross-over near by be- 

 tween Cc and Bb. 



