Early experiments suggested that outcrossing does not yield favorable results and thus is avoided to this day. No serious attempts are made to outcross among different breeds, which would take advantage of heterozygote advantage. In this case, however, essentially all crossing is performed within the same breed and usually involves inbreeding. 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. This in turn has given employment to several generations of teenagers who, in summer, detassel 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. This of course means that the hybrid "seed corn" must be produced each generation by crossing two distinct, inbred lines. Yields of hybrid corn are much greater than those of nonhybridized, "open-pollinated" varieties. Hybrid vigor has had a profound impact on agriculture. It is also known as hybrid vigor or, more commonly, as heterozygote advantage. This phenomenon had been known to farmers and plant breeders for many years, and was given the name "heterosis" by the U.S. Breeding with another variety ("outcrossing") produces offspring that are heterozygous for many genes, and is often accompanied by an increase in size and vigor. This is often accompanied by loss of vigor: slower growth, less resistance to disease, and other signs of decreased fitness. During this process, the organism becomes homozygous for many genes, meaning that its two gene copies are identical. Inbreeding is the practice of repeatedly crossing a single variety of organism with itself, in order to develop a more uniform variety. Heterozygote advantage also refers more narrowly to superior fitness of an organism that is heterozygous for a particular gene, usually one governing a disease. Fitness means the ability to survive and have offspring. A heterozygote is an organism with two different alleles, one donated from each parent. Heterozygote advantage is the superior fitness often seen in hybrids, the cross between two dissimilar parents.
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