DICENTRIC CHROMATIDS 



269 



5. Two loop chromatids and two fragments : with a chiasma 

 proximal to the inversion, which is disparate with respect to comple- 

 mentary chiasmata in the inversion . 



We thus see that pairs of crossings-over in the inversion may be 

 in a complementary, reciprocal or disparate relationship with one 

 another. On the other hand when crossing-over occurs proximal 

 to the inversion it may be in a comparate (complementary or 

 reciprocal) or disparate relationship with the crossing-over (single 

 or double) in the inversion. The comparate relationship makes no 

 observable difference to the effects of crossing-over in the inversion. 

 I'he disparate relationship has the effect of replacing first division 

 bridges by loops which will give second division bridges. 



Table 41A. 

 Results of Dyscentric Crossing-over 



(f, acentric chromatid ; b^ first division bridge ; b" second division 

 bridge from first division loop). 



^ Disparate crossing-over proximal to two disparate cross-overs in the 

 inversion is that where a chromatid, which had a cross-over in the inversion, 

 crosses over with one which had two, or none in the inversion. The other two 

 kinds of crossing-over are comparate. 



^ Where two loops are formed they sometimes interlock at the first division. 

 In this case the two second division bridges are interlocked unless they are 

 broken [Tulipa). 



It is difficult to observe the pachytene pairing of short inversions 

 directly and to determine the exact length of long ones, owing, as 

 we saw, to their pairing straight, wholly or in part, as though they 

 were normal. It is therefore desirable to estimate the length of an 

 inversion indirectly. For this purpose two assumptions can be 

 made, (i) Its length is not less than the proportion indicated by 

 the frequency of single chiasmata in it, as shown by the sum of 

 first and second division bridges, (ii) Its length is less than that of 



