Heterozygote advantage is the superior fitness often seen in hybrids, the cross between two dissimilar parents. A heterozygote is an organism with alleles particular forms two different alleles, one donated from each parent. Fitness means the abil-of genes ity to survive and have offspring. Heterozygote advantage also refers more narrowly to superior fitness of an organism that is heterozygous for a particular gene, usually one governing a disease.
Inbreeding is the practice of repeatedly crossing a single variety of organism with itself, in order to develop a more uniform variety. During this process, the organism becomes homozygous for many genes, meaning that its two gene copies are identical. This is often accompanied by loss of vigor: slower growth, less resistance to disease, and other signs of decreased fitness. This is known as inbreeding depression. 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 phenomenon had been known to farmers and plant breeders for many years, and was given the name "heterosis" by the U.S. geneticist George Shull in 1916. It is also known as hybrid vigor or, more commonly, as heterozygote advantage.
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