Increase in such characteristics as size, growth rate, fertility, and yield of a hybrid organism over those of its parents. Plant and animal breeders exploit heterosis by mating two different purebred lines that have desirable traits. The first-generation offspring generally show, in greater measure, the desired characteristics of both parents. Since this vigour may decrease if the hybrids are actually mated together, the parental lines must be maintained and crossed for each new crop or group desired.

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