Heterosis, or hybrid vigor, or outbreeding enhancement, is the improved or function of any biological quality in a hybrid offspring. It is the occurrence of a genetically superior offspring from mixing the genes of its parents.
Heterosis is the opposite of inbreeding depression. Inbreeding depression leads to offspring with deleterious traits due to homozygosity.
The term heterosis often causes controversy, particularly in selective breeding of domestic animals, because it is sometimes claimed that all crossbred plants and animals are genetically superior to their parents.
This is untrue, as only some hybrids are genetically superior. The inverse of heterosis, when a hybrid inherits traits from its parents that are not fully compatible, with deleterious results, is outbreeding depression.
Genetic Basis of Heterosis
Two competing hypotheses, not necessarily mutually exclusive, have been to explain hybrid vigor. The dominance hypothesis attributes the superiority of hybrids to the suppression of undesirable (deleterious) recessive alleles from one parent by dominant alleles from the other.
It attributes the poor performance of inbred strains to the loss of genetic diversity, with the strains becoming purely homozygous deleterious alleles at many loci.
The over dominance 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.
The concept of heterozygote advantage/overdominance is not restricted to hybrid lineages.
This hypothesis is commonly invoked to explain the persistence of many alleles (most famously the erythrocyte-sickling allele) that are harmful in homozygotes; in normal circumstances, such harmful alleles would be removed from a population through the process of natural selection.
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Like the dominance hypothesis, it attributes the poor performance of many inbred strains to a high frequency of these harmful recessive alleles and the associated high frequency of homozygous-recessive genotypes.
Hybrid Corn
Nearly all field corn (maize) grown in most developed nations exhibits. 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 out yielded 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 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 inbreeds.
They are generally more vigorous and also more uniform than the earlier double-cross hybrids. The process of creating these hybrids often involves detasseling.
Hybrid Livestock
The concept of heterosis is also applied in the production of commercial livestock. In cattle, hybrids between Black Angus and Hereford produce a hybrid 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 white eggs for sale in the United States.
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Heterosis Effect in Animals
Purebreds and inbreeds often carry genetic disease. Heterosis is a theory, where the phenomenon of crossing two inbred lines can produce descendants with superior genetic foundation.
In addition to the absence of inbreeding depressing, present in inbreed and purebred dogs in general, there is some remote inbreeding in any breed.
Heterosis is also produced by over dominance, i.e. better combined function of two diverse genes (alleles) on a gene site (locus), compared to two identical (but harmless) ones.
This increased health and vigor does not create a superior breed, but the advantages obtained from it are what produce hybrid vigor. This goal in this scenario is not to create a new breed, but to create a happy and healthy pet.
Heterosis effect results in a healthier, more vigorous dog with a reduced chance of genetic disease. It is well known in all domestic animal breeding, hybrids, 50%-50% mixes of two different breeds, will raise the chances of having less genetic diseases because all doubling of detrimental effects will stop in the first generation.
The genetic term for this is Heterosis Effect. This effect often gives non-related individuals stronger descendants than inbreeds.
Breeders who breed hybrid dogs have stated their goal was to get healthy and happy dogs without genetic problems. Most breeders crossing with the poodle are looking for a soft silky non-shedding coat good for allergy sufferers.
The purpose of these hybrids is not and should never be to develop a new breed. Once one goes beyond first generation purebred to purebred, you lose the heterosis effect, which is the goal for most hybrid breeders.
The mother should always be the bigger of the two, to avoid puppies getting too big and complicating the delivery for the mother. Heterosis is said to not only occur in the first generation, but also mating to a non-related hybrid of same (or other) type will also show this effect, though the aspect of the offspring will be different.
The hope is that the dogs will get the benefit of the greatly demanded Heterosis effect, and avoid genetic diseases which are common among purebreds and inbreeds.
In summary, Heterosisis the occurrence of a genetically superior offspring from mixing the genes of its parents.
The genetic explanation is that there is superiority of hybrids to the suppression of undesirable (deleterious) recessive alleles from one parent by dominant alleles from the other. It is found in plants, such as corn, and the production of commercial livestock.
In animals, heterosis results in a healthier, more vigorous with a reduced chance of genetic disease. Heterosis is the method of breeding desirable traits for high agricultural yield.
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