124 GENETICS IN RELATION TO AGRICULTURE 



TABLE XXVI. LINKAGE or VERMILION, SABLE, AND BAR IN DROSOPHILA 



FIG. 57. Diagram to illustrate interference in crossing-over. 



would protect the loci A, C, and D on either side of it from crossing-over 

 because there would be no close twisting of the chromosomes at these 

 points. As we move on toward E, however^the frequency of double 

 crossing-over would become greater and greater until at E where the 

 modal length of loop was attained double crossing-over values approach- 

 ing those expected on the basis of pure chance would be obtained. 

 Muller has actually shown that such conditions are fulfilled in his 

 twelve-point experiments and he has been able to plot a curve showing 

 that the observed frequency of double crossing-over gradually increases 

 until when the modal length is reached the curve coincides with that 

 based on pure chance. Thus we see again how another point of attack 

 has lent support to the conception that the factors are arranged in a 

 linear series and that the linkage relations of factors are referable to the 

 mechanical consequences of relative positions in the linear series. 



Bridges points out that interference stands in about the same relation 

 to linkage as linkage does to free Mendelian assortment. Also that the 

 development of the idea of interference is an illustration of the advantage 

 of the chromosome hypothesis. The existence of this phenomenon was 



