BASES OI- CHANGE 



with one another to give two new combinations. Mechanically this 

 reunion is just like that of crossing-over: physiologically, on the 

 other hand, it is something quite new. Since the breaks can occur 

 in any parts of any chromosomes, reunion can give a prodigious 

 variety of new combinations. It can occur between ends within one 

 arm of one chromosome, between the two arms of one chromosome, 

 or between the arms of two chromosomes, homologous or non- 

 homologous. Some of these new combinations arc unv/orkablc : 

 the new chromosomes have no centromere; or they have two 

 centromeres which act independently and so cause yet another 

 breakage by passing to opposite poles at anaphase. But other 

 combinations have only single centromeres and these are workable 

 and important (Fig. 20). 



The workable combinations arc of two kinds: an inversion happens 

 where the two breaks are in the same chromosome, an interchange 

 where they are in different chromosomes. Where they are in oppo- 

 site arms of the same chromosome we may look upon the change 

 cither as an interchange or an inversion. These types of reunion are 

 found wherever there is breakage. And, when the breakage is of 

 chromatids, the association of the changed and unchanged chroma- 

 tids remains at the following metaphase of mitosis to bear witness 

 to the course that events have followed. For example an interchange 

 of chromatids gives the appearance of a chiasma, and an inversion 

 of a chromatid gives a loop in addition to a chiasma (Fig. 30). 



Two chromosomes undergoing interchange may be represented 

 as of two segments each, AB and CD, which acquire the new order, 

 AD and BC or AC and BD. For inversion we need four such 

 segments to represent one chromosome, abed which changes into 

 acbd. The homozygous types derived from these changes show new 

 linkage relations. Experimental interchange in X-rayed Drosophila 

 moves blocks of genes from one linkage group to another. Inversion 

 leaves them in the same groups but their recombination frequencies 

 correspond to a new order on the map. 



Two breaks suffice to produce inversion or interchange. Three 

 will give more complicated changes. A piece may be taken out of 

 a chromosome and inserted or translocated wherever else the same 

 or another chromosome may be broken. Thus two chromosomes 

 iihc and def c^iu become ac and dbcj. In a succeeding generation this 



102 



