Hybrid vigor has had a profound impact on agriculture. Yields of hybrid corn are much greater than those of nonhybridized, "open-pollinated" varieties. This of course means that the hybrid "seed corn" must be produced each generation by crossing two distinct, inbred lines. This in turn has given employment to several generations of teenagers who, in summer, de-tassel corn plants, that is, remove the pollen from one of the parental plants to prevent self-pollination and assure cross-hybridization with the desired variety.
In domesticated animals used for meat or milk production, selection of the best producers to produce high-yielding offspring has been in effect for centuries and has produced dramatic results. In this case, however, essentially all crossing is performed within the same breed and usually involves inbreeding. No serious attempts are made to outcross among different breeds, which would take advantage of heterozygote advantage. Early experiments suggested that outcrossing does not yield favorable results and thus is avoided to this day.
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