Heterosis, hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions of its parents. These effects can be due to Mendelian or non-Mendelian inheritance.
In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, varieties, species or genera through sexual reproduction. Hybrids are not always intermediates between their parents, but can show hybrid vigour, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.
Mendelian inheritance is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900. These laws were initially controversial. When Mendel's theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics within the modern evolutionary synthesis.
Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel’s laws can be used to determine the distribution of phenotypes expected for the population of offspring. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values. Non-Mendelian Inheritance is applicable in co-dominance and incomplete dominance.
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.
George Harrison Shull was an eminent American plant geneticist and the younger brother of botanical illustrator and plant breeder J. Marion Shull. He was born on a farm in Clark County, Ohio, graduated from Antioch College in 1901 and from the University of Chicago (Ph.D.) in 1904, served as botanical expert to the Bureau of Plant Industry in 1903-04, and thenceforth was a botanical investigator of the Carnegie Institution at the Station for Experimental Evolution, Cold Spring Harbor, N. Y., giving special attention to the results of Luther Burbank's work.
The physiological vigor of an organism as manifested in its rapidity of growth, its height and general robustness, is positively correlated with the degree of dissimilarity in the gametes by whose union the organism was formed … The more numerous the differences between the uniting gametes — at least within certain limits — the greater on the whole is the amount of stimulation … These differences need not be Mendelian in their inheritance … To avoid the implication that all the genotypic differences which stimulate cell-division, growth and other physiological activities of an organism are Mendelian in their inheritance and also to gain brevity of expression I suggest … that the word 'heterosis' be adopted.
Heterosis is often discussed as the opposite of inbreeding depression although differences in these two concepts can be seen in evolutionary considerations such as the role of genetic variation or the effects of genetic drift in small populations on these concepts. Inbreeding depression occurs when related parents have children with traits that negatively influence their fitness largely due to homozygosity. In such instances, outcrossing should result in heterosis.
Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals. Population biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression.
Genetic variation is the difference in DNA among individuals. There are multiple sources of genetic variation, including mutation and genetic recombination.
Genetic drift is the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms. The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is the fraction of the copies of one gene that share a particular form. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation. It can also cause initially rare alleles to become much more frequent and even fixed.
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 depression.
In biology, outbreeding depression is when crosses between two genetically distant groups or populations results in a reduction of fitness. The concept is in contrast to inbreeding depression, although the two effects can occur simultaneously. Outbreeding depression is a risk that sometimes limits the potential for genetic rescue or augmentations. Therefore it is important to consider the potential for outbreeding depression when crossing populations of a fragmented species It is considered postzygotic response because outbreeding depression is noted usually in the performance of the progeny. Some common cases of outbreeding depression have arisen from crosses between different species or populations that exhibit fixed chromosomal differences.
Dominance versus overdominance is a scientific controversy in the field of genetics that has persisted for more than a century.These two alternative hypotheses were first stated in 1908.
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. For example, the connection between race and intelligence has been a subject of debate in both popular science and academic research since the inception of IQ testing in the early 20th century.
Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.
When a population is small or inbred, it tends to lose genetic diversity. Inbreeding depression is the loss of fitness due to loss of genetic diversity. Inbred strains tend to be homozygous for recessive 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 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 and overdominance have different consequences for the gene expression profile 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.
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 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.
"The current view ... is that the dominance hypothesis is the major explanation of inbreeding decline and [of] the high yield of hybrids. There is little statistical evidence for contributions from overdominance and epistasis. But whether the best hybrids are getting an extra boost from overdominance or favorable epistatic contributions remains an open question."
The term heterosis often causes confusion and even controversy, particularly in selective breeding of domestic animals, because it is sometimes (incorrectly) claimed that all crossbred plants and animals are "genetically superior" to their parents, due to heterosis[ citation needed ]. However, there are two problems with this claim:
An example of the ambiguous value judgements imposed on hybrids and hybrid vigor is the 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, making them "inferior" in terms of natural selection.
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)). 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 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.
An epigenetic contribution to heterosis has been established in plants,and it has also been reported in animals. MicroRNAs (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, a protein closely associated with DNA, which can either activate or repress associated genes.
One example of where particular genes may be important in vertebrate animals for heterosis is the major histocompatibility complex (MHC). Vertebrates inherit several copies of both MHC class I and MHC class II from each parent, which are used in 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. These genes are highly polymorphic throughout populations, but will be more similar in smaller, more closely related populations. Breeding between more genetically distant individuals will decrease the chance of inheriting two alleles which 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 and therefore a greater response. This will also mean that the immunity acquired to the pathogen will be against a greater range of antigens, meaning that the pathogen must mutate more before immunity is lost. Thus hybrids will be less likely to be succumb to pathogenic disease and will be more capable of fighting off infection.
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, sorghum, rice, sugar beet, onion, spinach, sunflowers, broccoli and to create a more psychoactive cannabis.
Nearly all field corn (maize) grown in most developed nations exhibits heterosis. Modern corn hybrids substantially outyield conventional cultivars and respond better to 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 of Michigan State University based on work begun in 1879 at the urging of Charles Darwin. Dr. Beal's work led to the first published account of a field experiment demonstrating hybrid vigor in corn, by Eugene Davenport and Perry Holden, 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 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. The process of creating these hybrids often involves detasseling.
Temperate maize hybrids are derived from two main heterotic groups: Iowa Stiff Stalk Synthetic, and non stiff stalk.[ citation needed ]
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 of ~15 tons per hectare. In India also, several varieties have shown high vigor, including RH-10 and Suruchi 5401.
The concept of heterosis is also applied in the production of commercial livestock. In cattle, crosses between Black Angus and Hereford produce a cross known as a "Black Baldy". In swine, "blue butts" are produced by the cross of Hampshire and Yorkshire. Other, more exotic hybrids such as "beefalo" are also used for specialty markets.
Within poultry, sex-linked 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 broilers 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 with a marketable carcass 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 for sale in the United States.
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.
In 2014, a study undertaken by the Centre for Integrative Ecology at Deakin University in Geelong, Victoria concluded that intraspecific hybrids between the subspecies flaveolus and elegans of the Crimson rosella (Platycercus elegans) were more likely to fight off diseases than their pure counterparts.
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 steady rise in IQ test scores around the world during the twentieth century.
An allele is a variant form of a given gene. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. A notable example of this trait of color variation is Gregor Mendel's discovery that the white and purple flower colors in pea plants were the result of "pure line" traits which could be used as a control for future experiments. However, most alleles result in little or no observable phenotypic variation.
Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic information of their parents. Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection. The study of heredity in biology is genetics.
Dominance, in genetics, is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked, and these show a very different inheritance and presentation pattern to autosomal traits which depends on the sex of the individual. Additionally, there are other forms of dominance such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.
Inbred strains are individuals of a particular species which are nearly identical to each other in genotype due to long inbreeding. A strain is inbred when it has undergone at least 20 generations of brother x sister or offspring x parent mating, at which point at least 98.6% of the loci in an individual of the strain will be homozygous, and each individual can be treated effectively as clones. Some inbred strains have been bred for over 150 generations, leaving individuals in the population to be isogenic in nature. Inbred strains of animals are frequently used in laboratories for experiments where for the reproducibility of conclusions all the test animals should be as similar as possible. However, for some experiments, genetic diversity in the test population may be desired. Thus outbred strains of most laboratory animals are also available, where an outbred strain is a strain of an organism that is effectively wildtype in nature, where there is as little inbreeding as possible.
Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J.B.S. Haldane, that states that if in a species hybrid only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex. The heterogametic sex is the one with two different sex chromosomes; in therian mammals, for example, this is the male.
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 locus is known as overdominance. Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals.
Heterosis is a condition in genetics where the phenotype of the heterozygote lies outside the phenotypical range of both homozygous parents. Overdominance can also be described as heterozygote advantage, wherein heterozygous individuals have a higher fitness than homozygous individuals.
An F1 Hybrid (also known as filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics, and in selective breeding, where it may appear as F1 crossbreed. The term is sometimes written with a subscript, as F1 hybrid. Subsequent generations are called F2, F3, etc.
Genetics, a discipline of biology, is the science of heredity and variation in living organisms.
In genetics, complementation occurs when two strains of an organism with different homozygous recessive mutations that produce the same mutant phenotype produce offspring with the wild-type phenotype when mated or crossed. Complementation will occur only if the mutations are in different genes. In this case, each strain's genome supplies the wild-type allele to "complement" the mutated allele of the other strain's genome. Since the mutations are recessive, the offspring will display the wild-type phenotype. A complementation test can be used to test whether the mutations in two strains are in different genes. Complementation will not occur if the mutations are in the same gene. The convenience and essence of this test is that the mutations that produce a phenotype can be assigned to different genes without the exact knowledge of what the gene product is doing on a molecular level. The complementation test was developed by American geneticist Edward B. Lewis.
In genetics, underdominance is the opposite of overdominance. It is the selection against the heterozygote, causing disruptive selection and divergent genotypes. Underdominance exists in situations where the heterozygotic genotype is inferior in fitness to either the dominant or recessive homozygotic genotype. Compared to examples of overdominance in actual populations, underdominance is considered more unstable and may lead to the fixation of either allele.
Out-crossing or out-breeding means the crossing between different breeds and no common ancestors. This is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of an individual being subject to disease or genetic abnormalities.
Polar overdominance is a unique form of inheritance originally described in livestock, with relevant examples in humans and mice being discovered shortly after. The term polar is used to describe this type of overdominance because the phenotype of the heterozygote is more prevalent than the other genotypes. This polarity is shown as differential phenotype is only present in one of the heterozygote configurations when the recessive allele is inherited in a parent of origin type fashion. Polar overdominance differs from regular overdominance where both heterozygote genotypes display a phenotype that has increased fitness regardless of the parent of origin. Studying this type of inheritance could have practical applications in preventative medicine for humans as well as a variety of other agricultural applications.
Edward Murray East was an American plant geneticist, botanist, agronomist and eugenicist. He is known for his experiments that led to the development of hybrid corn and his support of 'forced' elimination of the 'unfit' based on eugenic findings. He worked at the Bussey Institute of Harvard University where he performed a key experiment showing the outcome of crosses between lines that differ in a quantitative trait.
Zygosity is the degree of similarity of the alleles for a trait in an organism.
Genetic purging is the reduction of the frequency of a deleterious allele, caused by an increased efficiency of natural selection prompted by inbreeding.
The partial dominance hypothesis in genetics states that inbreeding depression is the result of the frequency increase of homozygous deleterious recessive or partially recessive alleles. The partial dominance hypothesis can be explained by looking at a population that is divided into a large number of separately inbred lines. Deleterious alleles will eventually be eliminated from some lines and become fixed in other lines, while some lines disappear because of fixation of deleterious alleles. This will cause an overall decline in population and trait value, but then increase to a trait value that is equal to or greater than the trait value in the original population. Crossing inbred lines restores fitness in the overdominance hypothesis and a fitness increase in the partial dominance hypothesis.
The shared evolutionary history of living humans has resulted in a high relatedness among all living people, as indicated for example by the very low fixation index (FST) among living human populations.
The massive efforts to study the human genome in detail have produced extraordinary amounts of genetic data. Although we still fail to understand the molecular bases of most complex traits, including many common diseases, we now have a clearer idea of the degree of genetic resemblance between humans and other primate species. We also know that humans are genetically very close to each other, indeed more than any other primates, that most of our genetic diversity is accounted for by individual differences within populations, and that only a small fraction of the species’ genetic variance falls between populations and geographic groups thereof.
Most studies of human population genetics begin by citing a seminal 1972 paper by Richard Lewontin bearing the title of this subsection . Given the central role this work has played in our field, we will begin by discussing it briefly and return to its conclusions throughout the chapter. ... A key conclusion of the paper is that 85.4% of the total genetic variation observed occurred within each group. That is, he reported that the vast majority of genetic differences are found within populations rather than between them. ... His finding has been reproduced in study after study up through the present: two random individuals from any one group (which could be a continent or even a local population) are almost as different as any two random individuals from the entire world