The following observations in phenotype and cytology of progeny suggested that actual exchange of chromosome segments was involved in genetic crossing over : (i) Association of knob in chromosome with the phenotype, colourless seed (c) and non-waxy endosperm (Wx) indicated crossing over in I region, because in the parent these were located on a knobless chromosome, (ii) At meiotic metaphase I, the presence of a ring of four chromosome without a knob suggested cytological exchange of chromosome segments, because in the parent the knob was associated with 9th chromosome carrying translocation (consult Structural Changes in Chromosomes and note that whenever a translocation is present in heterozygous condition, a ring of four chromosomes will be formed during meiosis). The classes 4th and 6th shown in the checkerboard in Fig. 10.17 will show this feature, (iii) Similarly, if there were no quadrivalents (ring of four) and only 10 bivalents were observed, the presence of knob in one of these bivalents could be treated as an evidence for cytological crossing over, since the knob was originally associated with translocation. This feature is exhibited by classes 3rd and 5th in Figure 10.17.
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