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
as II region, then one would expect two types of non-crossover gametes (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.