Heterosis - AbsoluteAstronomy.com

Heterosis, or hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. The adjective derived from heterosis is heterotic.

Heterosis is the occurrence of a superior offspring from mixing the genetic contributions of its parents. These effects can be due to Mendelian

Mendelian inheritance

Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...

or non-Mendelian inheritance

Non-mendelian inheritance

Non-Mendelian inheritance is a general term that refers to 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...

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 the opposite of inbreeding depression

Inbreeding depression

Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...

. Inbreeding depression leads to offspring with deleterious traits due to homozygosity

Zygosity

Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...

. 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

Outbreeding depression

A concept in selective breeding and zoology, outbreeding depression refers to cases when offspring from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population....

Genetic basis of heterosis

Two competing hypotheses, not necessarily mutually exclusive, have been developed 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 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

Heterozygote advantage

A heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...

. 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. 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.

In 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

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

, sorghum

Sorghum

Sorghum is a genus of numerous species of grasses, one of which is raised for grain and many of which are used as fodder plants either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents...

, rice

Rice

Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

, sugar beet

Sugar beet

Sugar beet, a cultivated plant of Beta vulgaris, is a plant whose tuber contains a high concentration of sucrose. It is grown commercially for sugar production. Sugar beets and other B...

, onion

Onion

The onion , also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion The onion...

, spinach

Spinach

Spinach is an edible flowering plant in the family of Amaranthaceae. It is native to central and southwestern Asia. It is an annual plant , which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions...

, sunflowers,broccoli

Broccoli

Broccoli is a plant in the cabbage family, whose large flower head is used as a vegetable.-General:The word broccoli, from the Italian plural of , refers to "the flowering top of a cabbage"....

and marijuana.

Corn (maize)

Nearly all field corn (maize

Maize

) grown in most developed nations

Developed country

A developed country is a country that has a high level of development according to some criteria. Which criteria, and which countries are classified as being developed, is a contentious issue...

exhibits heterosis. Modern corn hybrids substantially outyield conventional cultivars and respond better to fertilizer

Fertilizer

Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...

.

Corn heterosis was famously demonstrated in the early 20th century by George H. Shull

George Harrison Shull

George Harrison Shull was an eminent American plant geneticist. He was born in Clark Co., Ohio, graduated from Antioch College in 1901 and from the University of Chicago in 1904, served as botanical expert to the Bureau of Plant Industry in 1903-04, and thenceforth was a botanical investigator of...

and Edward M. East

Edward Murray East

Edward Murray East was an American plant geneticist, botanist and agronomist. He is known for his experiments that led to the development of hybrid corn. He worked at the Bussey Institute of Harvard University where he performed a key experiment showing the outcome of crosses between lines that...

after hybrid corn was invented by Dr. William James Beal of Michigan State University

Michigan State University

Michigan State University is a public research university in East Lansing, Michigan, USA. Founded in 1855, it was the pioneer land-grant institution and served as a model for future land-grant colleges in the United States under the 1862 Morrill Act.MSU pioneered the studies of packaging,...

based on work begun in 1879 at the urging of Charles Darwin

Charles Darwin

Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

. Dr. Beal's work led to the first published account of a field experiment

Field experiment

A field experiment applies the scientific method to experimentally examine an intervention in the real world rather than in the laboratory...

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

Donald F. Jones

Donald Forsha Jones was a United States maize geneticist and practical corn breeder at the Connecticut Agricultural Experiment Station, New Haven. Beginning at the station in 1914, he made high-yielding hybrid corn practical by his invention of the double-cross hybrid.In Jones' method, four...

at the Connecticut

Connecticut

Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...

Agricultural Experiment Station

Agricultural experiment station

An agricultural experiment station is a research center that conducts scientific investigations to solve problems and suggest improvements in the food and agriculture industry...

, 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

Detasseling

Detasseling corn is removing the pollen-producing flowers, the tassel, from the tops of corn plants and placing them on the ground. It is a form of pollination control, employed to cross-breed, or hybridize, two varieties of corn....

.

Temperate maize hybrids are derived from two main heterotic groups: Iowa Stiff Stalk Synthetic, often referred to as BSSS, and non stiff stalk.

Hybrid livestock

The concept of heterosis is also applied in the production of commercial livestock

Livestock

Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...

. In cattle, hybrids between Black Angus and Hereford

Hereford (cattle)

Hereford cattle are a beef cattle breed, widely used both in intemperate areas and temperate areas, mainly for meat production.Originally from Herefordshire, England, United Kingdom, more than five million pedigree Hereford Cattle now exist in over 50 countries...

produce a hybrid known as a "Black Baldy

Black Baldy

Black Baldy is a type of crossbred beef cattle produced by crossing Hereford cattle with a solid black breed, usually Aberdeen Angus. Angus bulls are also used on Hereford heifers in an attempt to produce smaller calves and reduce dystocia. The term is particularly used in Australia and New Zealand...

". In swine, "blue butts" are produced by the cross of Hampshire

Hampshire (pig)

The Hampshire pig is a domestic swine breed characterized by erect ears and a black body with a whitish band around the middle, covering the front legs....

and Yorkshire. Other, more exotic hybrids such as "beefalo

Beefalo

Beefalo are a fertile hybrid offspring of domestic cattle, Bos taurus, and the American bison, Bison bison...

" are also used for specialty markets.

Within poultry

Poultry

Poultry are domesticated birds kept by humans for the purpose of producing eggs, meat, and/or feathers. These most typically are members of the superorder Galloanserae , especially the order Galliformes and the family Anatidae , commonly known as "waterfowl"...

, 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

Egg (biology)

An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

for sale in the United States.

Humans

Experimental breeding of humans is considered unethical, so any evidence of heterosis in humans is derived from observational studies

Observational study

In epidemiology and statistics, an observational study draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator...

It has been suggested that many beneficial effects on average health, intelligence and height have resulted from an increased heterosis, in turn resulting from increased mixing of the human population such as by urbanization

Urbanization

Urbanization, urbanisation or urban drift is the physical growth of urban areas as a result of global change. The United Nations projected that half of the world's population would live in urban areas at the end of 2008....