Structural Changes in Chromosomes 



167 



PARACENTRIC BREAKS 

 ABCDEFGHIJ 



r 



A G H I J Deficiency 



C D 

 B C D E F or B C J E lost 



F 



(a) 



AFEDCBG HIJ 



Inversion 

 (b) 



PERICENTRIC BREAKS 

 ABCDEFGHIJ 



A B C D I J lost 



J Deficiency 



G F 



ABCDHGFEIJ 



Inversion 



(c) (d) 



FIGURE 12—2. Some consequences of two breaks in the same chromosome. 



is, to one side of the centromere, or peri- 

 centric, that is, with the centromere between 

 them (Figure 12-2). 



1. Deficiency 



Consider a chromosome linearly differen- 

 tiated as ABCDEFG.HIJ, the centromere 

 being between G and H. When the breaks 

 are paracentric in position (for example, 

 between A and B, and between F and G). 

 the fragments can unite to produce a centric 

 chromosome (AG. HIJ, Figure 12-2a) de- 

 ficient for the acentric interstitial (nonter- 

 minal) piece (BCDEF). The ends of the 

 latter fragment may join to produce a ring 



chromosome, or they may not. In either 

 event, the acentric fragment is usually lost 

 before the next nuclear division. When the 

 breaks are pericentric (for example, between 

 D and E, and between H and I ), the acentric 

 end pieces are lost, even if they join together 

 (Figure 12-2c). The middle centric piece 

 can survive if its ends join to form a ring 

 and if the deficient sections are not exten- 

 sive. Even if a ring survives because it is 

 not too hypoploid (the aneuploid condition 

 in which genes or chromosomal regions are 

 missing), it is still at a disadvantage because 

 a single crossing over either with a nonring 

 (rod) homolog or with another ring results 



