130 PHYSICAL BASIS OF HEREDITY 



The fact that interference reappears, i.e., that coinci- 

 dence decreases after reaching a maximum, indicates that 

 the segment of a chromosome between the breaking points 

 tends to be of a particular (modal) length; and that breaks 

 which are closer together or farther apart than this modal 

 length are less frequent. That is, genes not only stick 

 together in blocks, but the blocks tend to be of a definite 

 size, and longer and shorter blocks are less frequent. 

 In the sex-chromosome of Drosophila, which is 65 units 

 long, Weinstein's data indicate that the most frequent 

 length of block is about 46. In the second chromosome 

 (which is 107 units long). Bridges' data indicate a modal 

 length of about 15 in the centre of the chromosome and 

 of about 30 on either side of the middle point. 



The work on coincidence throws light on the behavior 

 of the chromosomes during crossing over. The cytologi- 

 cal evidence has not determined whether when crossing 

 over takes place the chromosomes are twisted loosely or 

 tightly. But MuUer has shown that this question may be 

 attacked by certain calculations based on the data of inter- 

 ference. If, as a rule, chromosomes twist in long loops, 

 crossing over at two points close together would be rare, 

 for it would require a shorter twist than usually occurs. 

 The occurrence of long loops would explain the interfer- 

 ence of neighboring regions. Moreover the decrease of 

 interference as distance increases would be accounted for, 

 because short loops would be less frequent than longer 

 ones. The reappearance of interference for widely separ- 

 ated regions is explained by supposing that extremely 

 long loops are infrequent as are very short ones. That is, 

 on the supposition of long twists there would be a modal 

 length of loop, and loops of greater or lesser length would 

 be less frequent. 



If, however, the chromosomes are tightly twisted into 

 short loops, the interference of neighboring regions might 

 be explained on the supposition that a break at one point 

 allows the chromosomes partly to unravel in the neighbor- 



