. Cell heredity. Cytogenetics. 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 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 The noncrossover strands are unaltered, but both crossover chromatids are lost: one because it lacks a cen
. Cell heredity. Cytogenetics. 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 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 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 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 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).. ^ 7 j^ r\ ClB ClB ClB + ClB Lethal Dies if lettial was induced by X-ray. Please note that these images are extracted from scanned page images that may have been digitally enhanced for re
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