1. By the use of intervarietal chromosome substitution lines, the poor crossability with rye (S. cereale, 2n = 14) of the wheat (T. aestivum, 2n = 42) variety Hope was shown to be determined by chromosomes 5A and 5B. The genes Kr1 and Kr2, which are responsbile for poor crossability (Lein, 1943), are probably located on chromosomes 5B and 5A respectively.
2. Crossability is actively inhibited by the dominant alleles, Kr1 and Kr2, of Hope and is apparently not enhanced by the recessive alleles, kr1 and kr2, of the readily crossable variety Chinese Spring.
3. The inhibition of crossability with rye conferred evolutionary and agricultural advantage upon wheat by preventing the production of sterile wheat-rye hybrids which could be regarded as weeds generated from within the crop.
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