Hybrids

In biology and specifically genetics, hybrid has several meanings, all referring to the offspring of sexual reproduction.
 * 1) In general usage, hybrid is synonymous with heterozygous: any offspring resulting from the mating of two distinctly homozygous individuals
 * 2) a genetic hybrid carries two different alleles of the same gene
 * 3) a structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities
 * 4) a numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes
 * 5) a permanent hybrid is a situation where only the heterozygous genotype occurs, because all homozygous combinations are lethal.

From a taxonomic perspective, hybrid refers to offspring resulting from the interbreeding between two animals or plants of different taxa.
 * 1) Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are sometimes known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids. Extremely rare interfamilial hybrids have been known to occur (such as the guineafowl hybrids). .   No interordinal (between different orders) animal hybrids are known.
 * 2) The second type of hybrid consists of crosses between populations, breeds or cultivars within a single species. This meaning is often used in plant and animal breeding, where hybrids are commonly produced and selected because they have desirable characteristics not found or inconsistently present in the parent individuals or populations. This flow of genetic material between populations or races is often called hybridization.

Etymology
According to the Oxford English Dictionary, the word is derived from Latin hybrida (or (h)ibrida), meaning the "offspring of a tame sow and a wild boar", "child of a freeman and slave", etc. The term entered into popular use in English in the 19th century, though examples of its use have been found from the early 17th century.

Types of hybrids
Depending on the parents, there are a number of different types of hybrids;

Single cross hybrids - result from the cross between two true breeding organisms and produces an F1 generation called an F1 hybrid (F1 is short for Filial 1, meaning "first offspring).  The cross between two different homozygous lines produces an F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. The F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other.

Double cross hybrids - result from the cross between two different F1 hybrids.

Three-way cross hybrids - result from the cross between one parent that is an F1 hybrid and the other is from an inbred line.

Triple cross hybrids - result from the crossing of two different three-way cross hybrids.

Population hybrids - result from the crossing of plants or animals in a population with another population. These include crosses between organisms such as interspecific hybrids or crosses between different races.

Interspecific hybrids
Interspecific hybrids are bred by mating two species, normally from within the same genus. The offspring display traits and characteristics of both parents. The offspring of an interspecific cross are very often sterile; thus, hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct. Sterility is often attributed to the different number of chromosomes the two species have, for example donkeys have 62 chromosomes, while horses have 64 chromosomes, and mules and hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes because the extra chromosome cannot make a homologous pair at meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.

Most often other processes occurring in plants and animals keep gametic isolation and species distinction. Species often have different mating or courtship patterns or behaviors, the breeding seasons may be distinct and even if mating does occur antigenic reactions to the sperm of other species prevent fertilization or embryo development. The Lonicera fly is the first known animal species that resulted from natural hybridization. Until the discovery of the Lonicera fly, this process was known to occur in nature only among plants.

While it is possible to predict the genetic composition of a backcross on average, it is not possible to accurately predict the composition of a particular backcrossed individual, due to random segregation of chromosomes. In a species with two pairs of chromosomes, a twice backcrossed individual would be predicted to contain 12.5% of one species' genome (say, species A). However, it may, in fact, still be a 50% hybrid if the chromosomes from species A were lucky in two successive segregations, and meiotic crossovers happened near the telomeres. The chance of this is fairly high: $$\frac{1}{2}^{(2 \times 2)} = \frac{1}{16}$$ (where the "two times two" comes about from two rounds of meiosis with two chromosomes); however, this probability declines markedly with chromosome number and so the actual composition of a hybrid will be increasingly closer to the predicted composition.

Hybrids are often named by the portmanteau method, combining the names of the two parent species. For example, a zeedonk is a cross between a zebra and a donkey. Since the traits of hybrid offspring often vary depending on which species was mother and which was father, it is traditional to use the father's species as the first half of the portmanteau. For example, a liger is a cross between a male lion and a female tiger, while a tigon is a cross between a male tiger and a female lion.

Examples of hybrid animals

 * Hybrid Iguana is single cross hybrid, result of natural inbreeding from male marine iguana and female land Iguana since late 2000s.
 * Equid hybrids
 * Mule, a cross of female horse and a male donkey.
 * Hinny, a cross between a female donkey and a male horse. Mule and Hinny are examples of reciprocal hybrids.
 * Zebroids
 * Zeedonk or Zonkey, a zebra/donkey cross.
 * Zorse, a zebra/horse cross
 * Zony or Zetland, a zebra/pony cross ("zony" is a generic term; "zetland" is specifically a hybrid of the Shetland pony breed with a zebra)
 * Bovid hybrids
 * Dzo, zo or yakow; a cross between a domestic cow/bull and a yak.
 * Beefalo, a cross of an American Bison and a domestic cow. This is a fertile breed; this along with genetic evidence has caused them to be recently reclassified into the same genus, Bos.
 * Zubron, a hybrid between Wisent (European Bison) and domestic cow.
 * Sheep-goat hybrids, such as the The Toast of Botswana.
 * Ursid hybrids, such as the Grizzly-polar bear hybrid, occur between black bears, brown bears, and polar bears.
 * Felid hybrids
 * Savannah cats are the hybrid cross between an African serval cat and a Domestic cat
 * A hybrid between a Bengal tiger and a Siberian tiger is an example of an intra-specific hybrid.
 * Ligers and Tiglons (crosses between a Lion and a Tiger) and other Panthera hybrids such as the Lijagulep. Various other wild cat crosses are known involving the Lynx, Bobcat, Leopard, Serval, etc.
 * Bengal cat, a cross between the Asian Leopard cat and the domestic cat, one of many hybrids between the domestic cat and wild cat species. The domestic cat, African wild cat and European wildcat may be considered variant populations of the same species (Felis silvestris), making such crosses non-hybrids.
 * Fertile Canid hybrids occur between coyotes, wolves, dingoes, jackals and domestic dogs.
 * Hybrids between Black Rhinos & White Rhinos have been recognized.
 * Hybrids between spotted owls and barred owls
 * Cama, a cross between a Camel and a Llama, also an intergeneric hybrid.
 * Wholphin, a fertile but very rare cross between a False Killer Whale and a Bottlenose Dolphin.
 * A fertile cross between an albino King Snake and an albino Corn Snake.
 * At Chester Zoo in the United Kingdom, a cross between African elephant (male) and Asian elephant (female). The male calf was named Motty. It died of gut infection after twelve days.
 * Cagebird breeders sometimes breed hybrids between species of finch, such as Goldfinch x Canary. These birds are known as Mules.
 * Gamebird hybrids, hybrids between gamebirds and domestic fowl, including Chickens, Guineafowl and Peafowl, interfamilial hybrids.
 * Numerous Macaw hybrids are also known.
 * Red Kite x Black Kite: 5 bred unintentionally at a falconry center in England. (It is reported that the black kite (the male) refused female black kites but mated with two female red kites.)
 * Hybridization between the endemic Cuban Crocodile (Crocodilus rhombifer) and the widely distributed American Crocodile (Crocodilus acutus) is causing conservation problems for the former species as a threat to its genetic integrity.
 * Blood parrot cichlid, which is probably created by crossing a Red Head Cihclid and a Midas Cichlid or Red Devil Cichlid
 * The Mulard duck, hybrid of the domestic Pekin duck and domesticated Muscovy ducks.
 * The Robert Ripley Museum houses a specimen from the VanEtten family which is claimed to be half elephant and half pig.

Hybrids should not be confused with chimaeras such as the chimera between sheep and goat known as the geep. Wider interspecific hybrids can be made via in vitro fertilization or somatic hybridization, however the resulting cells are not able to develop into a full organism. An example of interspecific hybrid cell lines is the humster (hamster x human) cells.

Hybrid plants
Plant species hybridize more readily than animal species, and the resulting hybrids are more often fertile hybrids and may reproduce, though there still exist sterile hybrids and selective hybrid elimination where the offspring are less able to survive and are thus eliminated before they can reproduce. A number of plant species are the result of hybridization and polyploidy with many plant species easily cross pollinating and producing viable seeds, the distinction between each species is often maintained by geographical isolation or differences in the flowering period. Since plants hybridize frequently without much work, they are often created by humans in order to produce improved plants. These improvements can include the production of more or improved; seeds, fruits or other plant parts for consumption, or to make a plant more winter or heat hardy or improve its growth and/or appearance for use in horticulture. Much work is now being done with hybrids to produce more disease resistant plants for both agricultural and horticultural crops. In many groups of plants hybridization has been used to produce larger and more showy flowers and new flower colors. Many plant genera and species have their origins in polyploidy. Autopolyploidy resulting from the sudden multiplication in the number of chromosomes in typical normal populations caused by unsuccessful separation of the chromosomes during meiosis. Tetraploids or plants with four sets of chromosomes are common in a number of different groups of plants and over time these plants can differentiate into distinct species from the normal diploid line. In Oenothera lamarchiana the diploid species has 14 chromosomes, this species has spontaneously given rise to plants with 28 chromosomes that have been given the name Oenthera gigas. Tetraploids can develop into a breeding population within the diploid population and when hybrids are formed with the diploid population the resulting offspring tend to be sterile triploids, thus effectively stopping the intermixing of genes between the two groups of plants (unless the diploids, in rare cases, produce unreduced gametes).

Another form of polyploidy called allopolyploidy occurs when two different species mate and produce hybrids. Usually the typical chromosome number is doubled in successful allopolyploid species, with four sets of chromosomes the genotypes can sort out to form a complete diploid set from the parent species, thus they can produce fertile offspring that can mate and reproduce with each other but can not back-cross with the parent species. Allopolyploidy in plants often gives them a condition called hybrid vigour, which results in plants that are larger and stronger growing than either of the two parent species. Allopolyploids are often more aggressive growing and can be invaders of new habitats.

Sterility in a hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, leaving them unable to produce chromosomally balanced gametes. While this is a negative in a crop such as wheat, when growing a crop which produces no seeds would be pointless, it is an attractive attribute in some fruits. Bananas and seedless watermelon, for instance, are intentionally bred to be triploid, so that they will produce no seeds. Many hybrids are created by humans, but natural hybrids occur as well.

Heterosis
Plant hybrids, especially, are often stronger than either parent variety, a phenomenon which when present is known as hybrid vigor (heterosis) or heterozygote advantage. Plant breeders make use of a number of techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize producing countries.

Examples of Species Hybrids
Some plant hybrids include:
 * Leyland Cypress, [X Cupressocyparis leylandii] hybrid between Monterey Cypress and Nootka Cypress.
 * Limequat, lime and kumquat hybrid.
 * Loganberry, a hybrid between raspberry and blackberry.
 * London Plane, a hybrid between Plantanus orientalis Oriental plane and Platanus occidentalis American plane (American sycamore). Thus forming [Plantanus x acerfolia]
 * Peppermint, a hybrid between spearmint and water mint.
 * Tangelo, a hybrid of a Mandarin orange and a pomelo or a grapefruit which may have been developed in Asia about 3,500 years ago.
 * Triticale, a wheat-rye hybrid.
 * Wheat; most modern and ancient wheat breeds are themselves hybrids. Bread wheat is a hexaploid hybrid of three wild grasses;  durrum (pasta) wheat is a tetraploid hybrid of two wild grasses.
 * Triangle of U: cabbage, mustard etc.

Some natural hybrids are:
 * White Flag Iris, a sterile hybrid which spreads by rhizome division
 * Evening primrose, a flower which was the subject of famous experiments by Hugo de Vries on polyploidy and diploidy.

Some horticultural hybrids:
 * Dianthus ×allwoodii, is a hybrid between Dianthus caryophyllus × Dianthus plumarius. This is an "interspecific hybrid" or hybrid between two species in the same genus.
 * ×Heucherella tiarelloides, or Heuchera sanguinea × Tiarella cordifolia is an "intergeneric hybrid" a hybrid between two different genera.
 * Quercus x warei [Quercus robur x Quercus bicolor] Kindred Spirit Hybrid Oak

Hybrids in nature
Hybridisation between two closely related species is actually a common occurrence in nature. Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation (Hybrid speciation). Recently DNA analysis of a bear shot by a hunter in the North West Territories confirmed the existence of naturally-occurring and fertile grizzly–polar bear hybrids. There have been reports of similar supposed hybrids, but this is the first to be confirmed by DNA analysis. In 1943, Clara Helgason described a male bear shot by hunters during her childhood. It was large and off-white with hair all over its paws. The presence of hair on the bottom of the feet suggests it was a natural hybrid of Kodiak and Polar bear.

In some species, hybridisation plays an important role in evolutionary biology. While most hybrids are disadvantaged as a result of genetic incompatibility, the fittest survive, regardless of species boundaries. They may have a beneficial combination of traits allowing them to exploit new habitats or to succeed in a marginal habitat where the two parent species are disadvantaged. This has been seen in experiments on sunflower species. Unlike mutation, which affects only one gene, hybridisation creates multiple variations across genes or gene combinations simultaneously. Successful hybrids could evolve into new species within 50-60 generations. This leads some scientists to speculate that life is a genetic continuum rather than a series of self-contained species.

Where there are two closely related species living in the same area, less than 1 in 1000 individuals are likely to be hybrids because animals rarely choose a mate from a different species (otherwise species boundaries would completely break down). In some closely related species there are recognized "hybrid zones".

Some species of Heliconius butterflies exhibit dramatic geographical polymorphism of their wing patterns, which act as aposematic signals advertising their unpalatability to potential predators. Where different-looking geographical races abut, inter-racial hybrids are common, healthy and fertile. Heliconius hybrids can breed with other hybrid individuals and with individuals of either parental race. These hybrid backcrosses are disadvantaged by natural selection because they lack the parental form's warning coloration, and are therefore not avoided by predators.

A similar case in mammals is hybrid White-Tail/Mule Deer. The hybrids don't inherit either parent's escape strategy. White-tail Deer dash while Mule Deer bound. The hybrids are easier prey than the parent species.

In birds, healthy Galapagos Finch hybrids are relatively common, but their beaks are intermediate in shape and less efficient feeding tools than the specialised beaks of the parental species so they lose out in the competition for food. Following a major storm in 1983, the local habitat changed so that new types of plants began to flourish, and in this changed habitat, the hybrids had an advantage over the birds with specialised beaks - demonstrating the role of hybridization in exploiting new ecological niches. If the change in environmental conditions is permanent or is radical enough that the parental species cannot survive, the hybrids become the dominant form. Otherwise, the parental species will re-establish themselves when the environmental change is reversed, and hybrids will remain in the minority.

Natural hybrids may occur when a species is introduced into a new habitat. In Britain, there is hybridisation of native European Red Deer and introduced Chinese Sika Deer. Conservationists want to protect the Red Deer, but the environment favors the Sika Deer genes. There is a similar situation with White-headed Ducks and Ruddy Ducks.

Expression of parental traits in hybrids
When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one parent has red flowers, the other has white, and the hybrid, pink flowers). Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other). Most characteristics of the typical hybrid are of one of these two types, and so, in a strict sense, are not really new. However, an intermediate trait does differ from those seen in the parents (e.g., the pink flowers of the intermediate hybrid just mentioned are not seen in either of its parents). Likewise, combined traits are new when viewed as a combination.

In a hybrid, any trait that falls outside the range of parental variation is termed heterotic. Heterotic hybrids do have new traits, that is, they are not intermediate. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids. Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while a tigon (lioness × tiger) is smaller. Also the hybrids between the Common Pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the Common Pheasant and hen Golden Pheasant (Chrysolophus pictus). Spurs are absent in hybrids of the former type, although present in both parents.

When populations hybridize, often the first generation (F1) hybrids are very uniform. Typically, however, the individual members of subsequent hybrid generations are quite variable. High levels of variability in a natural population, then, are indicative of hybridity. Researchers use this fact to ascertain whether a population is of hybrid origin. Since such variability generally occurs only in later hybrid generations, the existence of variable hybrids is also an indication that the hybrids in question are fertile.

Genetic mixing and extinction
Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing. Hybridization and introgression of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately effecting the population to such a degree that none of the originally genetically distinct population remains. .

Effect on biodiversity and food security
In agriculture and animal husbandry, the green revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity. Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.

Limiting factors
A number of conditions exist that limit the success of hybridization, the most obvious is great genetic diversity between most species. But in animals and plants that are more closely related hybridization barriers can include morphological differences, differing times of fertility, mating behaviors and cues, physiological rejection of sperm cells or the developing embryo.

In plants, barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.

Mythical and legendary hybrids
Ancient folktales often contain mythological creatures, sometimes these are described as hybrids (e.g. Hippogriff as the offspring of a griffin and a horse, and the Minotaur which is the offspring of Pasiphaë and a white bull. More often they are kind of chimera, i.e. a composite of the physical attributes of two or more kinds of animals, mythical beasts, and often humans, with no suggestion that they are the result of interbreeding, e.g. Harpies, mermaids and centaurs.