Heterosis

Heterotic string theory is a field of physics.

Heterosis is a term used in genetics and selective breeding. The term heterosis, also known as hybrid vigor, hybrid vigour, or outbreeding enhancement, describes the increased strength of different characteristics in hybrids; the possibility to obtain a "better" individual by combining the virtues of its parents.

Heterosis is often the opposite process of inbreeding depression, which increases homozygosity. The term often causes controversy, particularly in terms of the selective breeding of domestic animals, because it is sometimes believed that all crossbred plants or animals are better than their parents; this is not necessarily true. 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, typical examples of which are crosses between wild and hatchery fish that have incompatible adaptations. Heterosis can be classified into mid-parent heterosis, in which the hybrid shows increased strength which is greater than the average of both parents, and best-parent heterosis, in which the hybrid's increased strength is greater than that of the strongest parent. Mid-parent heterosis is more common in nature, and it is easier to explain (by mechanism of gene dominance; see below).

Genetic basis of heterosis

Two leading hypotheses explain the genetic basis for fitness advantage in 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. The heterozygous individual will then be able to express a broader array of parasite resistance alleles 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 heterozygote.

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. Mingroni (2004) has argued that heterosis may be partially responsible for the Flynn effect.

Flynn has criticized this. Breeding with close relatives is more likely to match unfavorable recessive genes which in turn can depress IQ. Therefore, a shift from inbreeding to outbreeding would raise IQ. If American history was a story of little isolated communities being replaced by a highly mobile society, that might help explain the massive IQ gains America has made throughout the 20th century. However, Americans never did live in small inbred groups. There was always a huge influx of migrants who settled in both urban and rural areas. There were huge population shifts during settlement of the West, after the Civil War, and during the World Wars. The growth of mobility has been modest: In 1870, 23 per cent of Americans were living in a state other than the one of their birth; in 1970, the figure was 32 percent (Mosler & Catley, 1998). Taking that trend as a rough measure of increased outbreeding, gives less than 3 percent of the population per generation. As for the beneficial effects of outbreeding, 3 IQ points is the average advantage of not breeding with one's cousins (Bouchard, 1998; Jensen, 1983).^[1]

Heterosis in fiction

The short story "Tonio Kröger" by Thomas Mann describes the fate of a man whose mother was South American 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.

In a famous Japanese anime series Dragonball Z, the hero Goku, an alien from a race called Saiyans, married a human, Chi Chi, and had two sons, Gohan and Goten which have mixed blood of saiyans and humans. Their children are depicted to have much greater power levels then their father.

The Marvel Comics super hero Namor the Sub-Mariner is the son of a human sea captain and of a princess of the mythical undersea kingdom of Atlantis. Namor is said to display the greatest super-strength and aquatic abilities of the "Homo mermanus" race in addition to other abilities attributed to his "hybrid vigor".

See also

• F1 hybrid
• Paternal effect

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
• Mingroni, M.A. (2004). "The secular rise in IQ: Giving heterosis a closer look". Intelligence 32: 65–83. 
• Nagoshi, C. T. & Johnson, R. C. (1986). "The ubiquity of g". Personality and Individual Differences 7: 201–207. 
• http://www.nwfsc.noaa.gov/publications/techmemos/tm30/lynch.htmlde:Heterosis-Effekt

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