No good source of resistance

This accumulation of horizontal resistance occurred without any good source of resistance. All the polygenes for horizontal resistance must have been present in the maize populations, but they obviously occurred at too low a frequency for the resistance to be expressed. If every individual host had only a few of the polygenes, the population as a whole would be highly susceptible. But every individual could have had different polygenes, and all the polygenes could then be present in the population. With each succeeding generation of recurrent mass selection in the presence of the rust, the percentage of polygenes in each individual would increase as a result of transgressive segregation. In this way, the resistance of the entire population would also increase.

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