Heterosis

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See also Hybrid vigor

Heterosis is a term used in genetics and selective breeding. The term heterosis, also known as hybrid vigor (or 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.

A mixed-breed dog

Heterosis is often the opposite process of inbreeding depression, which increases homozygosity. Although it is believed that heterosis is the action of many genes of small effect, whereas inbreeding depression is the action of a few genes of large effect. 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

See also: Hybrid vigor and Race and intelligence (test data)

Intelligence is associated with the degree of "hybrid vigor" (heterosis) manifested by offspring of cross-ethnic matings. Children of Japanese–Caucasian cross-ethnic matings score .26 SD higher on several cognitive tests than the average score of monoracial Japanese and Caucasian unions, despite having virtually identical parental educational and occupational backgrounds.^[1] (To clarify this point, we will use an example: If the average Caucasian offspring has an IQ of 100, and the average Japanese offspring has an IQ of 105, then the average Japanese-Caucasian cross-ethnic offspring will have an IQ of 104. (not the predicted score of 102.5)) The increase in scores tends to be higher on cognitive tests that are more g-loaded, suggesting that hybrid vigor improves g. Heterosis may be partially responsible for the Flynn effect.^[2]

However, correlation does not imply causation.

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. However, contrary to the popular belief, on average there is only 0.1% difference between the races, which implies that any the genomes of any two random humans are expected to differ by about 3 million base pairs. Of this 0.1% difference, 85% is found within any given population, 7 % is found between populations within a race and only 8% is found on average between the various races. Thus there is more genetic diversity within a race than between various races. ^[3] Therefore, breeding with a random individual of the same race will not match more unfavorable genes than breeding with a random individual of a different race. 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).^[4]

Others also criticized this. On average Ashkenazi Jews have IQs scoring approximately 107-115, but not all Jews (only Ashkenazi). ^[5] Ashkenazi Jews are usually overrepresented at Ivy League Schools. However, affirmative action programs usually ignore limiting the Ashkenazi Jewish quota, usually because of their white appearance, and Ashkenazi Jewish people are often mistaken as white. ^[6] Jewish-Asian marriages are quite common, but not necessary Ashkenazi. ^[7] However, because both Asians and Ashkenazi Jews are overrepresented at Ivy League universities, the Asians at Ivy League universities are likely to marry Ashkenazi Jews, not other Jews. Because of this, the Eurasian child on average may be more intelligent due to their part Ashkenazi Jewish ancestry, because the some of their white fathers may actually be Ashkenazi Jewish.

See also: Ashkenazi intelligence

Others also criticized this. A study showed that interracial parents, such as a white father and Asian mother, invest more time and money in their children than same-race parents. For example, children with two different race parents are more likely to participate in music, sports, etc. Eurasians are more likely to participate in sports because they are stereotyped to be as "strong as whites". (see #Stereotypes of people with partial Asian ancestry) Musical training at an early age increases IQ. (see Environment and intelligence) Since for their higher status, the children will be more likely to have higher IQ.^{[8] [9][10]}Template:Syn

Further criticism suggests the aversion of skin whitening products by Eurasians due to their "natural" light skin color. See Effects of neurotoxins in skin whitening products.

A conclusion might be that monoracial Japanese may have higher IQs if they do not use skin whitening products; hence the Eurasian score might simply be an "average" of Japanese and Caucasian scores. Template:Syn

Another simple explanation of increased IQ of Eurasians is diet differences. Eurasians are more likely to eat both a Western diet and Asian diet due to their parents' different cultures. Asian diets contain fish and vegetables. Western diets contain protein and other vitamins. Thus, Eurasians are more likely to get both the nutrients from the Asian and Western diets; therefore less likely to have micronutrient deficiency. Template:Syn

See also: Environment and intelligence

See also: Health and intelligence

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 inferior 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 proportional power levels then their father thru their percentage of Saiayan.

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 lowest super-strength and aquatic abilities of the "Homo mermanus" race in addition to other abilities attributed to his "hybrid vigor".

The central male character in the Underworld films derives his superior powers from heterosis.

See also

• F1 hybrid

References

Adapted from the Wikipedia article, "Heterosis" http://en.wikipedia.org/wiki/Heterosis, used under the GNU Free Documentation License.
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