In 1876, Charles Darwin reported that when he crossed two different true-breeding, homozygous genetic strains of corn, the offspring were 25 percent taller than either of the parent strains. Darwin’s observation was largely ignored for the next 30 years. In 1908, George Shull “rediscovered” this idea, reporting that not just plant height but the weight of the corn grain produced was dramatically higher in the offspring (Figure 12.12). Agricultural scientists took note, and Shull’s paper had a lasting impact on the field of applied genetics. The cultivation of hybrid corn spread rapidly in the United States and all over the world, quadrupling grain production. The practice of hybridization has spread to many other crops and animals used in agriculture. For example, beef cattle that are crossbred are larger and live longer than cattle bred within their own genetic strain.

Farmers have known for centuries that matings among close relatives (known as inbreeding) can result in inbreeding depression: reduced biological fitness arising from mating close relatives that tend to have the same recessive, sometimes deleterious, alleles. Inbreeding can produce offspring of smaller size and/or lower quality than matings between unrelated individuals. In contrast, superior qualities can arise from crossing inbred lines, a phenomenon called hybrid vigor, or heterosis (short for heterozygosis).

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The mechanism by which heterosis arises is not known but is important to understand, given how agriculture relies on heterosis for improving food production. Two competing hypotheses have been proposed. The dominance hypothesis explains the extra growth in hybrids as an absence of inbreeding depression—by the assumption that hybrids are unlikely to be homozygous for deleterious recessive alleles. The overdominance hypothesis postulates that in hybrids, new combinations of alleles from the parental strains interact with one another, resulting in superior traits that cannot occur in the parental lines. Because many of the characters seen in hybrid species are in fact controlled by multiple genes, recent studies suggest that both dominance and overdominance can contribute to heterosis in specific characters.