Using corn as the material, H.S. Creighton and Barbara McCIintock (1931) utilized the same principle which Stern utilized in case of fruitfly (Drosophila). They obtained a plant which had a knob on the 9th chromosome. This 9th chromosome was also involved in a reciprocal translocation with 8th chromosome (consult Structural Changes in Chromosomes). The plant was heterozygous for coloured aleurone and waxy endosperm characters and carried these genes in repulsion phase i.e., Cwx/cWx. Cwx was carried on the knobbed chromosome and cWx on the knobless chromosome, (c = recessive for colourless seed; wx = recessive for waxy endosperm). Such a plant was tcstcrossed with plants homozygous recessive for both characters i.e., colourless and waxy (cwx/cwx). If the chromosome region between the knob and c gene is represented as I region and that between c and Wx as II region, then one would expect two types of non-crossover gametes (Cwx and cWx) and six types of crossover gametes including single and double crossovers (Fig. 10.17). The progeny can be classified into eight types based on phenotypes and cytological observations.

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.