Chromosome knobs are enlargements on the chromosomes of corn (Zea mays L.) and its near relatives, visible on the threads at prophase of the first meiotic division. These enlargements stain deeply with carmine and vary in size from swellings but slightly more prominent than the adjacent chromomeres to those that equal the volume of a whole chromosome at anaphase. McClintock ^ was the first to report knobs on the midprophase chromosomes of corn. Since this first report, several investigators have found that knobs on the chromosomes of corn are seemingly a constant feature and aid materially in the identification of many of the 10 chromosomes. The writer was impressed by the fact that the location of the various knobs was not at random and that certain knobs are present in nearly all plants examined, whereas others are present in a very few plants. The location of the knobs and the unequal frequency with which they occur on the 20 arms of the chromosomes suggested the possibility that their frequency might be related to their position.

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