96 



Normal 



CELL HEREDITY 



9 il 2 3 4 sj 6 7 8 9 lo| 11 



17 



10 9 8 761 2345 11 

 Rearranged 9 o CX3 — 



17 



(a) 



17 



C^ 



Xl" 



-O-^S}-- 



(c) 



FIGURE 4.2. Origin of dicentric chromosome in maize (after B. McClintock, 1941, 

 Genetics, 26:234). 



(o). Comparison of chromosome 9 before and after rearrangement resulting from 

 three X-ray-induced breaks denoted by arrows. Stippled circles represent knobs, 

 clear circles represent centromeres, the dotted region 1-5 indicates position of cross- 

 over, and the heavy lines represent heterochromatic regions. 



(fa). Pachytene pairing of normal 9 with rearranged 9 showing point-to-point 

 pairing configuration, and the position of crossover which gave rise to the dicentric 

 chromosome shown in (c). 



(c). Dicentric chromosome resulting from the crossover shown in (fa). 



CHROMOSOME BREAKAGE AND REARRANGEMENTS 



Judging from the similarities in chromosome morphology and in 

 genetic linkage from strain to strain of most species, one might infer 

 that the chromosome is an exceedingly stable structure. It came as 

 something of a surprise, therefore, when geneticists found that chromo- 

 somes were readily able to undergo breakage and rearrangement. De- 



