THE MECHANICS OF THE CHROMOSOMES 



127 



that a chromosome may be pinched in between the two pairing threads 

 of another chromosome pair. If, in the formation of the diplotene 

 loops, sister chromosomes become separated by a spUt along the 

 reductional plane, this entangled chromosome should be pinched be- 

 tween the two paired chromatids on each side of the loop, but such 

 configurations are never found. Entangled chromosomes always lie 

 in the space between the two sides of the loop. It might, however, be 

 held that the presence of the entangled chromosome determines the 

 plane of separation in this region. If entangled chromosomes are found 

 on both sides of the middle chiasma in a bivalent with at least three 



Fig. 62. Chromosome Interlocking as a Demonstration of Crossing-Over. — 



The diagram shows the "classical" interpretation on the left; note that in one 

 region a chromosome would have to lie between sister chromatids, which is 

 impossible. The "chiasmatype" interpretation is in the middle, and an actual 

 example (in Eremurus) on the right. 



(From Mather, and Upcott.) 



chiasmata, it is clear from the above that the chiasma must involve a 

 cross-over.^ 



(b) It is argued that breakage of chiasmata does not occur. In some 

 organisms with Uttle terminaUzation (e.g. Lathyrus, Pisum) very little 

 or no reduction of chiasmata number occurs before metaphase, but 

 cross-over frequencies seem to be much the same as in organisms such 

 as Primula with complete terminaUzation and a great reduction in 

 chiasma number. Further, the statistical evidence suggests that only 

 sufficient chiasmata are formed to account for the observed cross-over 

 frequencies, and as not all these chiasmata become broken the number 

 of chiasma-breakages could not be sufficient to account for the amount 

 of crossing-over. (Cf. prediction of lengths of cross-over maps in 

 maize.) Moreover, the decrease in numbers of chiasmata between 

 diplotene and metaphase, when it occurs at all, is exactly paralleled by 

 the progress of terminahzation and can be explained by the pushing off 



* Mather 1933a. 



