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Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a
hybrid Hybrid may refer to: Science * Hybrid (biology), an offspring resulting from cross-breeding ** Hybrid grape, grape varieties produced by cross-breeding two ''Vitis'' species ** Hybridity, the property of a hybrid plant which is a union of two diff ...
offspring. An offspring is heterotic if its
traits Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language) * Trait the ...
are enhanced as a result of mixing the genetic contributions of its parents. These effects can be due to
Mendelian Mendelian inheritance is a type of biological inheritance Inheritance is the practice of passing on private property, titles A title is one or more words used before or after a person's name, in certain contexts. It may signify either ...

Mendelian
or
non-Mendelian inheritance Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in accordance with Mendel's laws Mendelian inheritance is a type of biology, biological inheritance (biology), inheritance that follows the principles o ...
.


Definitions

In proposing the term heterosis to replace the older term heterozygosis, G.H. Shull aimed to avoid limiting the term to the effects that can be explained by heterozygosity in Mendelian inheritance. Heterosis is often discussed as the opposite of
inbreeding depression Inbreeding depression is the reduced biological fitness Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual se ...
, although differences in these two concepts can be seen in evolutionary considerations such as the role of
genetic variation thumb File:Genetic Variation and Inheritance.svg, Parents have similar gene coding in this specific situation where they reproduce and variation in the offspring is seen. Offspring containing the variation also reproduce and passes down traits t ...

genetic variation
or the effects of
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical stru ...

genetic drift
in small populations on these concepts. Inbreeding depression occurs when related parents have children with
traits Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language) * Trait the ...
that negatively influence their fitness largely due to
homozygosity Zygosity (the noun, zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈɡ ...
. In such instances,
outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity Genetic diversity is the total number of gen ...
should result in heterosis. Not all outcrosses result in heterosis. For example, when a hybrid inherits traits from its parents that are not fully compatible, fitness can be reduced. This is a form of
outbreeding depressionIn biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, De ...
.


Dominance versus overdominance

Dominance versus overdominance is a
scientific controversy A scientific controversy is a substantial disagreement among scientists. A scientific controversy may involve issues such as the interpretation of data, which ideas are most supported by evidence, and which ideas are most worth pursuing. Controve ...
in the field of
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
that has persisted for more than a century. These two alternative hypotheses were first stated in 1908.


Genetic basis

When a population is small or inbred, it tends to lose genetic diversity.
Inbreeding depression Inbreeding depression is the reduced biological fitness Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual se ...
is the loss of fitness due to loss of genetic diversity. Inbred strains tend to be
homozygous Zygosity (the noun, zygote, is from the Greek zygotos "yoked," from zygon "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an or ...
for
recessive In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...

alleles
that are mildly harmful (or produce a trait that is undesirable from the standpoint of the breeder). Heterosis or hybrid vigor, on the other hand, is the tendency of outbred strains to exceed both inbred parents in fitness. Selective breeding of plants and animals, including hybridization, began long before there was an understanding of underlying scientific principles. In the early 20th century, after
Mendel's laws Mendelian inheritance is a type of biology, biological inheritance (biology), inheritance that follows the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 and popularized by William Bateson. These princi ...

Mendel's laws
came to be understood and accepted, geneticists undertook to explain the superior vigor of many plant hybrids. Two competing hypotheses, which are not mutually exclusive, were developed: * Dominance hypothesis. The dominance hypothesis attributes the superiority of hybrids to the suppression of undesirable recessive alleles from one parent by dominant alleles from the other. It attributes the poor performance of inbred strains to loss of genetic diversity, with the strains becoming purely homozygous at many loci. The dominance hypothesis was first expressed in 1908 by the geneticist
Charles Davenport Charles Benedict Davenport (June 1, 1866 – February 18, 1944) was a prominent biologist A biologist is a professional who has specialized knowledge in the field of biology, understanding the underlying mechanisms that govern the function ...
. Under the dominance hypothesis, deleterious alleles are expected to be maintained in a random-mating population at a selection–mutation balance that would depend on the rate of mutation, the effect of the alleles and the degree to which alleles are expressed in heterozygotes. * Overdominance hypothesis. Certain combinations of alleles that can be obtained by crossing two inbred strains are advantageous in the heterozygote. The overdominance hypothesis attributes the heterozygote advantage to the survival of many alleles that are recessive and harmful in homozygotes. It attributes the poor performance of inbred strains to a high percentage of these harmful recessives. The overdominance hypothesis was developed independently by Edward M. East (1908) and George Shull (1908). Genetic variation at an overdominant locus is expected to be maintained by balancing selection. The high fitness of heterozygous genotypes favours the persistence of an allelic polymorphism in the population. Dominance and overdominance have different consequences for the of the individuals. If overdominance 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 dominance is the cause, fewer genes should be 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 fitter of the two parents.


Historical retrospective

Population geneticist James Crow (1916–2012) believed, in his younger days, that overdominance was a major contributor to hybrid vigor. In 1998 he published a retrospective review of the developing science. According to Crow, the demonstration of several cases of
heterozygote advantage A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. The specific case of heterozygote advantage due to a single loc ...
in Drosophila and other organisms first caused great enthusiasm for the overdominance theory among scientists studying plant hybridization. But overdominance implies that yields on an inbred strain should decrease as inbred strains are selected for the performance of their hybrid crosses, as the proportion of harmful recessives in the inbred population rises. Over the years, experimentation in plant genetics has proven that the reverse occurs, that yields increase in both the inbred strains and the hybrids, suggesting that dominance alone may be adequate to explain the superior yield of hybrids. Only a few conclusive cases of overdominance have been reported in all of genetics. Since the 1980s, as experimental evidence has mounted, the dominance theory has made a comeback. Crow wrote:
The current view ... is that the dominance hypothesis is the major explanation of inbreeding decline and the high yield of hybrids. There is little statistical evidence for contributions from overdominance and
epistasis File:Epistasis.png, Example of epistasis in coat colour genetics: If no pigments can be produced the other coat colour genes have no effect on the phenotype, no matter if they are dominant or if the individual is homozygous. Here the genotype "c ...

epistasis
. But whether the best hybrids are getting an extra boost from overdominance or favorable epistatic contributions remains an open question.


Controversy

The term heterosis often causes confusion and even controversy, particularly in selective breeding of
domestic animal This page gives a list of domestic animals, also including a list of animals which are or may be currently undergoing the process of domestication and animals that have an extensive relationship with humans beyond simple predation. This includes s ...
s, because it is sometimes (incorrectly) claimed that all
crossbred A crossbreed is an organism with purebred parents of two different breeds, varieties, or populations. ''Crossbreeding'', sometimes called "designer crossbreeding", is the process of breeding such an organism, While crossbreeding is used to maint ...
plants and animals are "genetically superior" to their parents, due to heterosis,. but two problems exist with this claim: * First, according to an article published in the journal ''
Genome Biology ''Genome Biology'' is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self-regulation by qualified members of a profession w ...
'', " genetic superiority" is an ill-defined term and not generally accepted terminology within the scientific field of genetics. A related term fitness is well defined, but it can rarely be directly measured. Instead, scientists use objective, measurable quantities, such as the number of seeds a plant produces, the germination rate of a seed, or the percentage of organisms that survive to reproductive age. From this perspective,
crossbred A crossbreed is an organism with purebred parents of two different breeds, varieties, or populations. ''Crossbreeding'', sometimes called "designer crossbreeding", is the process of breeding such an organism, While crossbreeding is used to maint ...
plants and animals exhibiting heterosis may have "superior" traits, but this does not necessarily equate to any evidence of outright "genetic superiority". Use of the term "superiority" is commonplace for example in crop breeding, where it is well understood to mean a better-yielding, more robust plant for agriculture. Such a plant may yield better on a farm, but would likely struggle to survive in the wild, making this use open to misinterpretation. In human genetics any question of "genetic superiority" is even more problematic due to the historical and political implications of any such claim. Some may even go as far as to describe it as a questionable value judgement in the realm of politics, not science. * Second, not all hybrids exhibit heterosis (see
outbreeding depressionIn biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, De ...
). An example of the ambiguous value judgements imposed on hybrids and hybrid vigor is the
mule A mule is the of a male (jack) and a female (). Horses and donkeys are different species, with different numbers of s. Of the two between these two species, a mule is easier to obtain than a , which is the offspring of a female donkey () a ...

mule
. While mules are almost always infertile, they are valued for a combination of hardiness and temperament that is different from either of their horse or donkey parents. While these qualities may make them "superior" for particular uses by humans, the infertility issue implies that these animals would most likely become extinct without the intervention of humans through
animal husbandry Animal husbandry is the branch of agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Ex ...
, making them "inferior" in terms of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
.


Genetic and epigenetic bases

Since the early 1900s, two competing genetic hypotheses, not necessarily mutually exclusive, have been developed to explain hybrid vigor. More recently, an epigenetic component of hybrid vigor has also been established. The genetic dominance hypothesis attributes the superiority of hybrids to the masking of expression of undesirable (deleterious) recessive alleles from one parent by dominant (usually wild-type) alleles from the other (see
Complementation (genetics) In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
). It attributes the poor performance of inbred strains to the expression of homozygous deleterious recessive alleles. The genetic overdominance hypothesis states that some combinations of alleles (which can be obtained by crossing two inbred strains) are especially advantageous when paired in a heterozygous individual. This hypothesis is commonly invoked to explain the persistence of some alleles (most famously the
Sickle cell trait Sickle cell trait describes a condition in which a person has one abnormal allele of the hemoglobin beta gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Men ...
allele) that are harmful in homozygotes. In normal circumstances, such harmful alleles would be removed from a population through the process of natural selection. Like the dominance hypothesis, it attributes the poor performance of inbred strains to expression of such harmful recessive alleles. In any case, outcross matings provide the benefit of masking deleterious recessive alleles in progeny. This benefit has been proposed to be a major factor in the maintenance of sexual reproduction among eukaryotes, as summarized in the article
Evolution of sexual reproduction Sexual reproduction is an adaptive feature which is common to almost all multi-cellular organisms (and also some single-cellular organisms) with many being incapable of asexual reproduction, reproducing asexually. Prior to the advent of sexual ...
. An epigenetic contribution to heterosis has been established in plants, and it has also been reported in animals.
MicroRNA A microRNA (abbreviated miRNA) is a small single-stranded non-coding RNA A non-coding RNA (ncRNA) is an RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or materi ...
s (miRNAs), discovered in 1993, are a class of non-coding small RNAs which repress the translation of messenger RNAs (mRNAs) or cause degradation of mRNAs. In hybrid plants, most miRNAs have non-additive expression (it might be higher or lower than the levels in the parents). This suggests that the small RNAs are involved in the growth, vigor and adaptation of hybrids. 'Heterosis without hybridity' effects on plant size have been demonstrated in genetically isogenic F1 triploid (autopolyploid) plants, where paternal genome excess F1 triploids display positive heterosis, whereas maternal genome excess F1s display negative heterosis effects. Such findings demonstrate that heterosis effects, with a genome dosage-dependent epigenetic basis, can be generated in F1 offspring that are genetically isogenic (i.e. harbour no heterozygosity). It has been shown that hybrid vigor in an allopolyploid hybrid of two ''Arabidopsis'' species was due to epigenetic control in the upstream regions of two genes, which caused major downstream alteration in chlorophyll and starch accumulation. The mechanism involves acetylation and/or methylation of specific amino acids in
histone H3 Histone H3 is one of the five main histones involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal end, N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'be ...
, a protein closely associated with DNA, which can either activate or repress associated genes.


Major histocompatibility complex in animals

One example of where particular genes may be important in vertebrate animals for heterosis is the
major histocompatibility complex The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are ca ...
(MHC). Vertebrates inherit several copies of both
MHC class I MHC class I molecules are one of two primary classes of major histocompatibility complex The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell su ...
and
MHC class II MHC Class II molecules are a class of major histocompatibility complex The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essen ...
from each parent, which are used in
antigen presentation Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented antigens displayed on Cell membrane, cell surfaces, antigen processing must occur before the antigen ...

antigen presentation
as part of the adaptive immune system. Each different copy of the genes is able to bind and present a different set of potential peptides to
T-lymphocytes A T cell is a type of lymphocyte A lymphocyte is a type of white blood cell in the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ...
. These genes are highly polymorphic throughout populations, but are more similar in smaller, more closely related populations. Breeding between more genetically distant individuals decreases the chance of inheriting two alleles that are the same or similar, allowing a more diverse range of peptides to be presented. This, therefore, gives a decreased chance that any particular pathogen will not be recognised, and means that more antigenic proteins on any pathogen are likely to be recognised, giving a greater range of T-cell activation, so a greater response. This also means that the immunity acquired to the pathogen is against a greater range of antigens, meaning that the pathogen must
mutate A tulip flower exhibiting a partially yellow petal due to a mutation in its genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...
more before immunity is lost. Thus, hybrids are less likely to succumb to pathogenic disease and are more capable of fighting off infection. This may be the cause, though, of autoimmune diseases.


Plants

Crosses between inbreds from different heterotic groups result in vigorous F1 hybrids with significantly more heterosis than F1 hybrids from inbreds within the same heterotic group or pattern. Heterotic groups are created by plant breeders to classify inbred lines, and can be progressively improved by reciprocal recurrent selection. Heterosis is used to increase yields, uniformity, and vigor. Hybrid breeding methods are used in
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It can also be ...

maize
,
sorghum ''Sorghum'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circum ...

sorghum
,
rice Rice is the seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was relea ...

rice
,
sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose Sucrose is a type of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated ...
,
onion The onion (''Allium cepa'' L., from Latin ''cepa'' "onion"), also known as the bulb onion or common onion, is a vegetable Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still co ...

onion
,
spinach Spinach (''Spinacia oleracea'') is a leafy LEAFY (abbreviated LFY) is a plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy int ...

spinach
,
sunflowers ''Helianthus'' () is a genus comprising about 70 species of annual and perennial flowering plants in the daisy family Asteraceae. Except for three South American species, the species of ''Helianthus'' are native to North America and Central Amer ...

sunflowers
,
broccoli Broccoli (''Brassica oleracea'' var. ''italica'') is an edible green plant in the Brassicaceae, cabbage family (family Brassicaceae, genus ''Brassica'') whose large Pseudanthium, flowering head, plant stem, stalk and small associated leafy green ...

broccoli
and to create a more
psychoactive A psychoactive drug, psychopharmaceutical, psychoactive agent, or psychotropic drug, is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...
cannabis ''Cannabis'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including ...

cannabis
.


Corn (maize)

Nearly all field corn (
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It can also be ...

maize
) grown in most
developed nations A developed country (or industrialized country, high-income country, more economically developed country (MEDC), advanced country) is a sovereign state A sovereign state is a political entity A polity is an identifiable political entity ...
exhibits heterosis. Modern corn hybrids substantially outyield conventional cultivars and respond better to
fertilizer A fertilizer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

fertilizer
. Corn heterosis was famously demonstrated in the early 20th century by George H. Shull and Edward M. East after hybrid corn was invented by Dr.
William James Beal William James Beal (March 11, 1833 – May 12, 1924) was an American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America ...
of
Michigan State University Michigan State University (MSU) is a public In public relations Public relations (PR) is the practice of managing and disseminating information from an individual or an organization An organization, or organisation (Englis ...
based on work begun in 1879 at the urging of
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
. Dr. Beal's work led to the first published account of a
field experiment Field experiments are experiments carried out outside of laboratory settings. They randomization, randomly assign subjects (or other sampling units) to either treatment or control groups in order to test claims of causality, causal relationships ...
demonstrating hybrid vigor in corn, by Eugene Davenport and , 1881. These various pioneers of botany and related fields 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 Connecticut () is the southernmost state in the New England region of the United States. As of the 2010 United States census, 2010 Census, it has the highest per-capita income, second-highest level of List of U.S. states and territories by H ...
Agricultural Experiment StationAn agricultural experiment station (AES) or agricultural research station (ARS) is a scientific research center#REDIRECT Center {{Redirect category shell, 1= {{Redirect from alternative spelling {{Redirect from ambiguous term ... that investigates ...
,
New Haven New Haven is a coastal city in the U.S. state of Connecticut. It is located on New Haven Harbor on the northern shore of Long Island Sound in New Haven County, Connecticut, and is part of the New York City metropolitan area. With a population ...
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. The process of creating these hybrids often involves detasseling. Temperate maize hybrids are derived from two main heterotic groups: 'Iowa Stiff Stalk Synthetic', and nonstiff stalk.


Rice (''Oryza sativa'')

Rice production has seen enormous rise in China due to heavy uses of hybrid rice. In China, efforts have generated a super hybrid rice strain ('LYP9') with a production capability around 15 tons per hectare. In India also, several varieties have shown high vigor, including 'RH-10' and 'Suruchi 5401'.


Hybrid livestock

The concept of heterosis is also applied in the production of commercial
livestock Livestock are the domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to secure a more predictabl ...
. In cattle, crosses between Black Angus and
Hereford Hereford () is a cathedral city City status in the United Kingdom is granted by the monarch of the United Kingdom The monarchy of the United Kingdom, commonly referred to as the British monarchy, is the constitutional monarchy of the ...
produce a cross known as a "
Black Baldy Image:Black Baldy.jpg, 250px, A Black Baldy cow and calf Black Baldy is a type of crossbred beef cattle produced by crossing Hereford (cattle), Hereford cattle with a solid black breed, usually Aberdeen Angus. Angus bulls are also used on Hereford ...

Black Baldy
". In swine, "blue butts" are produced by the cross of
Hampshire Hampshire (, ; abbreviated to Hants) is a Counties of England, county in South East England on the coast of the English Channel. The county town is Winchester, but the county is named after Southampton. Its two largest cities are Southampton a ...
and Yorkshire. Other, more exotic hybrids (two different species, so genetically more dissimilar), such as "
beefalo Beefalo constitute a fertile Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quant ...

beefalo
", are also used for specialty markets.


Poultry

Within
poultry Poultry () are domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to secure a more predictable ...

poultry
,
sex-linked Sex linked describes the sex-specific patterns of inheritance Inheritance is the practice of passing on private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations ...
genes have been used to create hybrids in which males and females can be sorted at one day old by color. Specific genes used for this are genes for barring and wing feather growth. Crosses of this sort create what are sold as Black Sex-links, Red Sex-links, and various other crosses that are known by trade names. Commercial
broiler A broiler is any chicken The chicken (''Gallus gallus domesticus'') is a domestication, domesticated subspecies of the red junglefowl originally from Southeastern Asia. Rooster or cock is a term for an adult male bird, and a younger male ...
s are produced by crossing different strains of White Rocks and White Cornish, the Cornish providing a large frame and the Rocks providing the fast rate of gain. The hybrid vigor produced allows the production of uniform birds at a marketable carcass weight at 6–9 weeks of age. Likewise, hybrids between different strains of White Leghorn are used to produce laying flocks that provide the majority of white
eggs Egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do ...
for sale in the United States.


Dogs

In 2013, a study found that mixed breeds live on average 1.2 years longer than pure breeds. John Scott and John L. Fuller performed a detailed study of purebred Cocker Spaniels, purebred Basenjis, and hybrids between them. They found that hybrids ran faster than either parent, perhaps due to heterosis. Other characteristics, such as basal heart rate, did not show any heterosis—the dog's basal heart rate was close to the average of its parents—perhaps due to the additive effects of multiple genes. Sometimes people working on a dog-breeding program find no useful heterosis. All this said, studies do not provide definitive proof of hybrid vigor in dogs. This is largely due to the unknown heritage of most mixed breeds mutts used. Results vary wildly, with some studies showing benefit and others finding the mixed breed dogs to be more prone to genetic conditions.


Birds

In 2014, a study undertaken by the Centre for Integrative Ecology at
Deakin University Deakin University is a public university #REDIRECT Public university #REDIRECT Public university #REDIRECT Public university#REDIRECT Public university A public university or public college is a university or college that is in state ownershi ...
in Geelong, Victoria, concluded that intraspecific hybrids between the subspecies ''Platycercus elegans flaveolus'' and ''P. e. elegans'' of the crimson rosella (''P. elegans'') were more likely to fight off diseases than their pure counterparts.


Humans

Human beings are all extremely genetically similar to one another. Michael Mingroni has proposed heterosis, in the form of hybrid vigor associated with historical reductions of the levels of inbreeding, as an explanation of the
Flynn effect The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century. When intelligence quotient (IQ) tests are initially stan ...

Flynn effect
, the steady rise in IQ test scores around the world during the 20th century.


See also

*
F1 hybrid An F1 hybrid (also known as filial 1 hybrid) is the first filial generation A generation is "all of the people born and living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic o ...
*
Genetic admixture Genetic admixture occurs when previously diverged or isolated genetic lineages mix.⅝ Admixture results in the introduction of new genetic lineages into a population. Examples Climatic cycles facilitate genetic admixture in cold periods and gene ...
*
Heterozygote advantage A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. The specific case of heterozygote advantage due to a single loc ...


References


Further reading

* *
NOAA Tech Memo NMFS NWFSC-30: Genetic Effects of Straying of Non-Native Hatchery Fish into Natural Populations: Inbreeding Depression and Outbreeding Depression
*

an article from University of Illinois Extension's ''Home Hort Hints'' * Roybal, J. (July 1, 1998).
Ranchstar
. ''Beef'' (beefmagazine.com).

regarding poultry; at FeatherSite {{Authority control Breeding Classical genetics Plant sexuality