This term means that progeny may have a higher level of an inherited, quantitative variable than either of their parents. Breeding for horizontal resistance depends very heavily on this phenomenon.
Suppose there is a population of wild potatoes in which every individual plant possesses only 10% of the polygenes contributing to horizontal resistance. The population as a whole will be highly susceptible, and so will every individual within that population. But suppose also the every individual possesses a different combination polygenes in its 10% share. This means that all the polygenes are present in the population, but no individual possesses enough of them to be resistant. With random cross-pollination, some of the progeny will have more than 10% of those polygenes (and others will have less). And, given adequate selection pressures, this percentage will increase with each successive generation. After enough generations of cross-pollination, there will be progenies that have most, possibly all, of the polygenes. These individuals will be highly resistant. And their resistance will be durable.
This breeding process of artificially increasing a variable character by transgressive segregation is called recurrent mass selection.
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