CYTOGENETIC CORRELATIONS AND CROSSING OVER 



103 



The presence of a heterozygous inversion is characteristically seen as 

 an inversion loop in paired homologous chromosomes, as shown in Fig- 

 ure 4.5. If no crossover occurs within the loop, there are no aberrant 

 genetic or cytological consequences. If a crossover does occur, there 

 are definite consequences. 



The cytogenetic consequences of crossing over within a paracentric 

 inversion loop are shown in Figure 4.5c. The noncrossover strands are 

 unaltered, but both crossover chromatids are lost: one because it lacks 

 a centromere, cannot orient on the spindle, and is eliminated; and the 

 other because it has two centromeres which move to opposite poles pro- 

 ducing an anapha.se bridge which finally breaks at random, giving rise 

 to two incomplete chromatids. This effective elimination of crossover 

 chromatids results in the apparent absence of crossing over. Thus inver- 

 sions were originally detected as crossover suppressors and were utilized 

 as such for a long time before their cytological basis was established. 



The most celebrated use of a crossover suppressor was in Muller's 

 famous ClB stock of Drosophila, which he developed for the detection 



X-ray treatment 

 of male 



FIGURE 4.6. The ClB method of 

 Muller. The chromosome marked ClB 

 contains a long inversion, a domi- 

 nant mutant gene 6 (not present on 

 the Y chromosome) and a recessive 

 lethal, /. X-rayed males are mated 

 with ClB females, and in the Fj gen- 

 eration, only ClB females ore saved. 

 Each is mated individually and the 

 progeny scored for the presence of 

 males. If no males are present, it 

 may be inferred that a lethal was in- 

 duced in the irradiated X chromo- 

 some (after Sturtevant and Beadle, 

 1939, An Infroduction fo Genetics, 

 Philadelphia, Saunders, p. 209). 



ClB 



ClB 



ClB + 



