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 a genetic hybrid carries two different alleles of the same gene a structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities a numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes 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[2]. 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).[3]. No interordinal (between different orders) animal hybrids are known. 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.[4] 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.[5] Types of hybrids Depending on the parents, there are a number of different types of hybrids;[6] 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.[7] Three-way cross hybrids - result from the cross between one parent that is an F1 hybrid and the other is from an inbred line.[8] 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.[9] 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.[10] 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: (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 A "Zeedonk", a zebra/donkey hybrid A "Liger", a Lion/Tiger hybrid A "Jaglion", a Jaguar/Lion hybrid 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 Tigons (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. [2] 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. 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. A sterile Trillium hybrid between Trillium cernuum and Trillium grandiflorum

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