LINKAGE, CROSSING-OVER, AND THE GERM PLASM 293 



"There is a corollary of importance to this conclusion. When a 

 cross is made that involves only white and bar, the double crossing- 

 over, that can be detected only when an intermediate point is followed, 

 must still be supposed to take place. Whenever it does take place 

 white bar flies and red round flies result. These will be added to the 

 non-crossover classes since they have the same external character- 

 istics. Consequently, the non-crossover classes will be increased and 

 the crossover classes decreased. In fact, the sum of the two cross- 

 over percentages 33 and 22 (55) is much greater than the apparent 

 amount (44) of crossing-over when only bar and white are involved. 

 Here then we have an explanation of why long distances taken as a 

 whole give too little crossing-over, as compared with the same distances 

 taken section by section. The lowered percentage is an actual ne- 

 cessity owing to the occurrence of double crossing-over." 



Interference. — One of the neatest confirmations of the crossover 

 hypothesis is one that was first advanced by Muller. According to 

 this idea, whenever double crossing-over occurs, two points of crossing- 

 over cannot be near together unless the chromosomes coil rather 

 tightly about each other. Consequently, if crossing-over occurs at a 

 given point there cannot be further crossing-over at the same time at 

 nearby points. Now it has been actually demonstrated that this is 

 true, for genes in the neighborhood of a crossed-over gene do not 

 themselves cross-over with that gene in the degree expected on the 

 basis of the law of probability. This is supposed to be a sort of inter- 

 ference with free crossing-over. "It may be supposed," says Castle, 

 "that chromosomes are somewhat like sticks of candy. Break one in 

 two at one point and it is unlikely that a break will occur simulta- 

 neously within a short distance of the first break, the strain there being 

 already relieved." The fact that what must be expected in this case 

 is actually realized in every case goes far toward establishing the 

 validity of the whole crossing-over hypothesis and the use of this 

 hypothesis in showing the linear arrangement of the genes in the 

 chromosomes. 



In this chapter it is hardly feasible to present any further evidence in 

 support of the claims of the Drosophila school of geneticists that they 

 have actually discovered the inmost secrets about the finer details of 

 the mechanism of Mendelian heredity. Suffice it to say that the evi- 

 dence is voluminous and consistent. Not a single fact has come to 

 light which is incompatible with the hypothesis, and new facts are 

 continually coming to light that agree with the hypothesis and lend it 



