In genetics , plant breeding and animal breeding, the heterosis effect describes the particularly pronounced performance of hybrids (mixed breeds), for example of the offspring of two different types of plants or animal breeds . A heterosis effect is used when the observed performance of the first branch generation (F 1 ) is higher than the average performance of the parent varieties or races (parental generation, parent generation).
The parents of the hybrids are homozygous for the traits examined , but the two parents are different from each other. The offspring of the F 1 generation, the heterotic hybrids, are mixed-breed ( heterozygous ) and identical to one another (homogeneous), according to Mendel's first rule ( rule of uniformity). This is based on the fact that in the double set of chromosomes one allele comes from the mother and one from the father. If the two parents are true breeds in many traits, but differ greatly from the other parent, the result is a hybrid F 1 generation that can be bred again and again , whose characteristics differ from both parents
Due to the genetically different pure-bred breeding lines of the parental generation (= parent generation), it is achieved during the crossing that many alleles of the crossing parents are different. Strongly heterozygous organisms have more different genetic makeup than purebred ones. They are often more resistant to disease and can adapt better to changing environmental conditions. Disadvantageous, recessive properties are also not (or hardly) realized in the phenotype of the hybrid.
Hybrid breeding is mainly used to increase fertility traits, which normally have a low heritability , i.e. are poorly hereditary (fertility in pigs, seed yield in cultivated plants).
Hybrid breeding is used for B. bees, pigs, hybrid chickens and in crop production (cereals, maize).
According to Mendel's second rule , however, the mixed inheritance decreases from the second branch generation (F 2 ): each self-fertilization of hybrids reduces the degree of heterozygosity (and thus usually the degree of heterosis of the traits) by half. Mild inbreeding, such as continued sibling mating in animals, lowers the degree of heterozygosity less sharply, but after many generations it also lowers it to zero. If the offspring of hybrids reproduce by cross-fertilization, as is usual with animals, then they reach a more normal state on this inbred hybrid scale and are then neither inbred nor hybrid. If, however, the offspring of hybrids, as is usual with many plants (e.g. barley), reproduce by self-fertilization, then with the generations they lose their mixed inheritance, they become pure-breeding. You lose everything that was present in the hybrid vigor in the original hybrid. However, the heterosis effect in such plants is rather small from the start, so they lose little over this generation sequence.
Note: If the two parents are hybrids themselves, then they themselves realize heterosis and, for heterotic reasons, do not have to be less productive than their crossbreed offspring.
Three-way hybrids (inbred line 1 x inbred line 2) x inbred line 3 are also used in maize breeding .
The heterosis effect in cereal species such as maize or rye can double (and more) the yields compared to such parents ( inbred lines ). Here, however, the previous inbreeding depression in higher inbreeding generations (..., I6) of the parents must be taken into account. From the perspective of vigorous hybrids one recognizes in the short stature of inbred lines their inbred depression; from the point of view of these inbred lines, the heterosis (hybrid growth, hybrid growth) of the hybrids (hybrids). The proportion of hybrid varieties has risen sharply in the last few decades. In addition to the high performance, the ability to plan the breeding result with the help of genomics and the protection against reproduction by the farmers are of particular importance. In 1995, broccoli , tomatoes and Brussels sprouts were all hybrid varieties with over 80 percent of the varieties.
Heterosis in humans
Intelligence researchers like Michael Mingroni consider heterosis as the cause of the steady increase in human intelligence, the so-called Flynn effect .
The heterosis effect could also lead to greater attractiveness in humans. In several studies, people of mixed race were clearly overrepresented among the best-looking faces.
The German botanist Joseph Gottlieb Kölreuter provided a first description of this phenomenon as early as 1766. In his studies of tobacco and thorn apple he observed that the offspring of the crosses showed greater vigor than the parent plants and that there was a connection between the strength of this phenomenon and the differences between the parents.
The term heterosis was proposed by the plant geneticist George Harrison Shull in 1914 at lectures in Göttingen, where he derived it from heteros and osis .
In 2002, the German Research Foundation (DFG) established the priority program Heterosis in order to contribute to the causal analysis of this key biological phenomenon with plant genome research and to develop the basis for its optimal use in plant breeding.
As part of the topic of green genetic engineering , the DFG President announced on May 13, 2009 at a press conference of the DFG and DLG that a. the molecular causes of heterosis are to be elucidated with the help of genetic engineering methods.
In September 2009, the University of Hohenheim hosted a three-day international conference Heterosis in Plants: Genetic and molecular causes and optimal exploitation in breeding .
The five-year ZUCHTWERT project started on September 1, 2014 : Basics of breeding methods for harnessing heterosis in wheat varieties . It was funded by the Federal Ministry of Food and Agriculture with a total of approx. 5 million euros, which was distributed among several project partners. The Leibniz Institute for Plant Genetics and Crop Plant Research (IPK) in Gatersleben will receive 655,370 euros . The State Plant Breeding Institute of the University of Hohenheim receives 370,775 euros for this, making ZUCHTWERT a research focus at the University of Hohenheim. The remaining approx. 3.9 million euros were distributed among sixteen groups, companies and organizations that deal with wheat breeding in Germany.
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