INTERLOCKING 



257 



away from the point of interlocking in both, one or neither of the 

 pairs, thus (Fig. 83) : — 



(i) Proximal Interlocking. The chiasmata move away from the 

 point of interlocking in both pairs of chromosomes, owing to its 

 being within the centromere loop. 



(ii) Distal Interlocking. The chiasmata on either side of the 

 interlocking move in the same direction, i.e., towards one end, in 



Fig. 84. — The genetical interpretation of double interlocking. 

 Right, pachytene, and left, diplotene. Crossing-over must have 

 occurred at the critical chiasma. (After Mather, 1932.) 



both of the interlocked pairs, owing to the interlocking being distal 

 to the most proximal chiasma. 



(iii) Proximal-distal Interlocking. The interlocking is proximal 

 as to one pair and distal as to the other. 



The distinction between this type and the second cannot be 

 expected to be made in practice. Both will appear, in an organism 

 with complete terminalisation, indistinguishable from a multiple 

 chiasma except in the most favourable material. They may both 

 therefore be described as distal. 



These kinds of interlocking should be found between separate 

 pairs of chromosomes and between separate chromosomes of 

 multiple rings, in interchange heterozygotes and polyploids. 



A classification of the available observations based on these 

 views shows that all the types that are expected on the 

 chiasmatype hypothesis are found, and none that are not 

 expected. 



R. A. CYTOLOGY. 



