Over the years, experimentation in plant genetics has proven that the reverse occurs, that yields increase in both the inbred strains and the hybrids, suggesting that dominance alone may be adequate to explain the superior yield of hybrids. But overdominance implies that yields on an inbred strain should decrease as inbred strains are selected for the performance of their hybrid crosses, as the proportion of harmful recessives in the inbred population rises. According to Crow, the demonstration of several cases of heterozygote advantage in Drosophila and other organisms first caused great enthusiasm for the overdominance theory among scientists studying plant hybridization. Population geneticist James Crow, who in his younger days believed that overdominance was a major contributor to hybrid vigor, has undertaken a retrospective review of the developing science. It attributes the poor performance of inbred strains to a high percentage of these harmful recessives. The overdominance hypothesis attributes to heterozygote advantage the survival of many alleles that are recessive and harmful in homozygotes. Certain combinations of alleles that can be obtained by crossing two inbred strains are advantageous in the heterozygote. It attributes the poor performance of inbred strains to loss of genetic diversity, with the strains becoming purely homozygous at many loci. The dominance hypothesis attributes the superiority of hybrids to the suppression of undesirable recessive alleles from one parent by dominant alleles from the other. This phenomenon is known as inbreeding depression. Inbred strains tend to be homozygous for recessive alleles that are mildly harmful (or produce a trait that is undesirable from the standpoint of the breeder). Two competing hypotheses, which are not mutually exclusive, were developed: In the early 20th century, after Mendel's laws came to be understood and accepted, geneticists undertook to explain the superior vigor of many plant hybrids. Selective breeding of plants and animals, including hybridization, began long before there was an understanding of underlying scientific principles.
When a population is small or inbred, it tends to lose genetic diversity.
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