Heterosis
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Heterosis is increased strength of different characteristics in hybrids; the possibility to obtain a "better" individual by combining the virtues of its parents.
This is commonly known as hybrid vigor or outbreeding enhancement. It is often the opposite process of inbreeding depression, which increases homozygosity. Heterosis is an example of heterozygous advantage. The term often causes controversy, particularly in terms of domestic animals, because it is sometimes believed that all crossbred plants or animals are better than their parents; this is untrue. Rather, when a hybrid is seen to be superior to its parents, this is known as hybrid vigor. It may also happen that a hybrid inherits such different traits from their parents that make them unfit for survival. This is known as outbreeding depression, and usually occurs in breeding between wild fish and hatchery fish that have incompatible adaptations.
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Genetic basis of heterosis
There are two leading hypothesis explaining the genetic basis behind the fitness advantage of heterosis.
The overdominance hypothesis implies that the combination of divergent alleles at a particular locus will result in a higher fitness in the heterozygote than in the homozygote. Take the example of parasite resistance controlled by gene A, with two alleles A and a, both expressing functional antigens. The heterozygous individual will then be able to express a broader array of antigens (from alleles A and a) and thus resist a broader array of parasites. The homozygous individual, on the other hand, will only express one allele of gene A (either A or a) and therefore will not resist as many parasites as the heterozygous.
The second hypothesis involves avoidance of deleterious recessive genes (also called the general dominance hypothesis), such that heterozygous individuals will express less deleterious recessive alleles than its homozygous counterpart.
The two hypotheses will have different consequences on the gene expression profile of the individuals. If over-dominance is the main cause for the fitness advantages of heterosis, then there should be an over-expression of certain genes in the heterozygous offspring compared to the homozygous parents. On the other hand, if avoidance of deleterious recessive genes is the cause, then there should be fewer genes that are under-expressed in the heterozygous offspring compared to the parents. Furthermore, for any given gene, the expression should be comparable to the one observed in the best of the two parents.
Hybrid corn
Nearly all the field corn now grown in the United States and most other developed nations is hybrid corn. Modern corn hybrids substantially outyield conventional cultivars and respond better to fertilization.
Heterosis in maize was first demonstrated in the early 20th Century by George H. Shull and Edward M. East. They showed that crosses of inbred lines made from a Southern dent and a Northern flint, respectively, showed substantial heterosis and outyielded conventional cultivars of that era. However, at that time such hybrids could not be economically made on a large scale for use by farmers. Donald F. Jones at the Connecticut Agricultural Experiment Station, New Haven invented the first practical method of producing a high-yielding hybrid maize in 1914-1917. Jones' method produced a double-cross hybrid, which requires two crossing steps working from four distinct original inbred lines. Later work by corn breeders produced inbred lines with sufficient vigor for practical production of a commercial hybrid in a single step, the single-cross hybrids. Single-cross hybrids are made from just two original parent inbreds. They are generally more vigorous and also more uniform than the earlier double-cross hybrids.
Heterosis in humans
One study suggests that hybrid vigor may be associated with increased intelligence in humans. Nagoshi and Johnson (1986) found that children of Japanese-Caucasian cross-ethnic unions scored .26 SD higher on several cognitive tests than those from within-ethnic families, despite having virtually identical parental educational and occupational backgrounds (p. 204). The increase in scores tended to be higher on cognitive tests that were more g-loaded, suggesting that hybrid vigor improves g. To become more reliable, the study would have to match the intelligence averages of the parents as well as compute the intelligence scores of their children. Mingroni (2004) has argued that heterosis may be partially responsible for the Flynn effect.
The Minnesota Transracial Adoption Study shows US children of African-Caucasian cross-ethnic unions score .47 SD lower on cognitive tests taken at age 17 than adopted children of Caucasian heritage, and .67 SD higher than adopted children of African heritage. (The average IQ of the adopting parents in this study was more than 1 standard deviation above the population mean of 100.) The exact interpretation of this study is disputed.
Heterosis in fiction
The short story "Tonio Kröger" by Thomas Mann describes the fate of man whose mother was Italian in a closed-minded North German town. Tonio Kröger is described as having superior characteristics because of the fusion of physical attributes and cultures, as well as superior intelligence. Ultimately, he is forced to leave his hometown due to the intolerance of his neighbours.
William Faulkner's Light in August character Joe Christmas can also be seen as a comment on hybrid vigor.
See also
References
- HYBRID VIGOR IN PLANTS AND ITS RELATIONSHIP TO INSECT POLLINATION - a section from Insect Pollination Of Cultivated Crop Plants by S.E. McGregor, USDA
- F1 Hybrids General Discussion - professor's notes from an Oregon State University course
- Hybrids & Heirlooms - an article from University of Illinois Extension
- Template:Cite journal
- Template:Cite journal
- http://www.nwfsc.noaa.gov/publications/techmemos/tm30/lynch.htmlde:Heterosis-Effekt