question: Why can some animals cross breed between them, while others cannot. For example, donkeys and horses can breed to give a mule, but nothing comes out if your breed humans and chimpanzees.
comment: There may be different reasons. For humans and chimpanzees we have differences in the chromosomes that don't allow us to breed. it's like trying to plug a USB cable into a micro USB port, they're similar, but the differences don't allow for the mix. Now, with donkeys and horses they can have a mule, but consider that the mule can't breed, it's sterile. So while some animals can breed, they aren't meant to, and are still to some extent incompatible.
passages: Hybridization between members of different but related genera is sometimes possible, as in the case of cama, wholphin, and some felid hybrids, for example.
  
  Section::::Reports of attempted hybridization.
  
  There have been no scientifically verified specimens of a human–chimp hybrid, but there have been substantiated reports of unsuccessful attempts at human/chimpanzee hybridization in the Soviet Union in the 1920s, and various unsubstantiated reports on similar attempts during the second half of the 20th century.
  
  Ilya Ivanovich Ivanov was the first person to attempt to create a human–chimp hybrid by artificial insemination.
   This degree of chromosomal similarity is roughly equivalent to that found in equines. Interfertility of horses and donkeys is common, although sterility of the offspring (mules) is nearly universal (with only around 60 exceptions recorded in equine history). Similar complexities and prevalent sterility pertain to horse–zebra hybrids, or zorses, whose chromosomal disparity is very wide, with horses typically having 32 chromosome pairs and zebras between 16 and 23 depending on species. In a direct parallel to the chimp–human case, the Przewalski's horse ("Equus ferus przewalskii") with 33 chromosome pairs, and the domestic horse ("E. f. caballus") with 32 pairs, have been found to be interfertile, and produce semi-fertile offspring: male hybrids can breed with female domestic horses.
   BULLET::::- Plants: Two species of Monkeyflowers, "mimulus lewsii" and "mimulus cardinalis," were found to have highly specialized pollinators that acted on major genes resulting in a contribution to the floral evolution and reproductive isolation of these two species. The specialized pollination limited gene flow between the two species, eventually resulting in two different species.
  
  BULLET::::- Sika deer: Sika deer were introduced into Western Europe, and they reproduce easily with the native red deer. This translocation of Sika deer has led to introgression and there are no longer "pure" red deer in the region, and all can be classified as hybrids.
   Hinnies and mules are hybrids resulting from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus "Equus", but "Equus caballus" has 64 chromosomes, while "Equus asinus" only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not cross. In order to obtain mules or hinnies it is necessary to train the progenitors to accept copulation between the species or create them through artificial insemination.
   Section::::Genetic control over sexual mimicry.
  
  Some organisms’ sexual mimicry is genetically determined by specific alleles. Unlike sexual mimicry that arises due to molecular compounds or hormones and can sometimes be induced through these molecules, this sexual mimicry arises from the organism’s genetic material. Besides the female hyenas’ sexual anatomy, which is part of their genetics, some other organisms have only some males/females in their population who look like the opposite sex and this is determined by specific alleles.
   A hybrid has normal viability but is typically deficient in terms of reproduction or is sterile. This is demonstrated by the mule and in many other well known hybrids. In all of these cases sterility is due to the interaction between the genes of the two species involved; to chromosomal imbalances due to the different number of chromosomes in the parent species; or to nucleus-cytoplasmic interactions such as in the case of "Culex" described above.
   Section::::Artificial speciation.
  
  New species have been created by animal husbandry, but the dates and methods of the initiation of such species are not clear. Often, the domestic counterpart of the wild ancestor can still interbreed and produce fertile offspring as in the case of domestic cattle, that can be considered the same species as several varieties of wild ox, gaur, yak, etc., or domestic sheep that can interbreed with the mouflon.
   In cattle, there are systems of crossbreeding. In many crossbreeds, one is larger than the other. One is used when the purebred females are particularly adapted to a specific environment, and are crossed with purebred bulls from another environment to produce a generation having traits of both parents.
  
  Section::::Crossbreeds in specific animals.:Sheep.
  
  The large number of breeds of sheep, which vary greatly, creates an opportunity for crossbreeding to be used to tailor production of lambs to the goal of the individual stockman.
  
  Section::::Crossbreeds in specific animals.:Llamas.
   Animals with homogeneous appearance, behavior, and other characteristics are known as particular breeds, and they are bred through culling animals with particular traits and selecting for further breeding those with other traits. Purebred animals have a single, recognizable breed, and purebreds with recorded lineage are called pedigreed. Crossbreeds are a mix of two purebreds, whereas mixed breeds are a mix of several breeds, often unknown. Animal breeding begins with breeding stock, a group of animals used for the purpose of planned breeding. When individuals are looking to breed animals, they look for certain valuable traits in purebred stock for a certain purpose, or may intend to use some type of crossbreeding to produce a new type of stock with different, and, it is presumed, superior abilities in a given area of endeavor. For example, to breed chickens, a breeder typically intends to receive eggs, meat, and new, young birds for further reproduction. Thus, the breeder has to study different breeds and types of chickens and analyze what can be expected from a certain set of characteristics before he or she starts breeding them. Therefore, when purchasing initial breeding stock, the breeder seeks a group of birds that will most closely fit the purpose intended.
   The Culex "pipiens pipiens" bioform of "Culex pipiens" usually inhabits areas above ground and targets avian species. The Culex "pipiens molestus" bioform, on the other hand, usually inhabits underground cavities and targets human hosts. When these two bioforms hybridize, the progeny shows depolarization in both habitat and host preferences. As a result, these hybridized species play a role in disease transmission. This is because avian species amplify diseases such as West Nile virus. A hybridized mosquito can become infected by feeding on an avian amplifier and then feed on a human, effectively passing on West Nile pathogens to that human. 
   Chimpanzees and humans are closely related (sharing 95% of their DNA sequence and 99% of coding DNA sequences; visual chart: ), leading to contested speculation that a hybrid is possible. The closest known data is that hybridization between chimpanzees and bonobos, which share 99.6% of the genome ( and see the chart) is easily possible (). Some authors even say that "the population split between bonobo and chimpanzee occurred relatively close in time to the split between the bonobo–chimpanzee ancestor (Pan ancestor) and humans", or that Pan, especially bonobos , are a "living fossil" close to our ancestors, "Pan prior". However, genetic similarity and thus the chances of successful hybridization can be different from visual similarity; for example, pugs and huskies look quite dissimilar (perhaps more so than some chimpanzees or bonobos or even gorillas are from humans) but belong to the same species and subspecies and can hybridize freely. 
   The offspring of the mutated mouse with the inbred strain has a 50% chance of carrying the mutation. From this, the offspring with the mutation can be mated to a heterozygous carrier which then creates offspring with 75% of the genetic background. This backcrossing is then continued until more than 99% is genetic background and the mutated locus is inherited. However, if the specific mutation cannot be passed on, heterozygous animals should be used to breed with the original inbred strain. Full-sib mating are used to maintain coisogenic strains if the specific gene locus is homozygous. However, a regular backcrossing of these coisogenic strains with their standard parental strain is preferred in order to avoid subline divergence.
   Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile, due to the two different sets of chromosomes being unable to pair up during meiosis. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridization in creating new species of animals is unclear, although cases have been seen in many types of animals, with the gray tree frog being a particularly well-studied example.
   BULLET::::- Black sex-link like "Black rocks" are a cross between unique specially bred hybrid strains of Rhode Island Red rooster (but any non-white and non barred rooster may be used for other black sex-link crosses) and a Barred Rock hen (which carry both extended black and barring genes).
  
  BULLET::::- Red sex-links are a cross between a Rhode Island Red or New Hampshire rooster and a White Rock (This variety pair is known as a Golden Comet), Silver Laced Wyandotte, Rhode Island White, or Delaware hen.
   This is particularly true for the marmoset. Recent research shows most marmosets are chimeras, sharing DNA with their fraternal twins. 95% of marmoset fraternal twins trade blood through chorionic fusions, making them hematopoietic chimeras.
   Reinforcement, sometimes referred to as the Wallace effect, is the process by which natural selection increases reproductive isolation. It may occur after two populations of the same species are separated and then come back into contact. If their reproductive isolation was complete, then they will have already developed into two separate incompatible species. If their reproductive isolation is incomplete, then further mating between the populations will produce hybrids, which may or may not be fertile. If the hybrids are infertile, or fertile but less fit than their ancestors, then there will be further reproductive isolation and speciation has essentially occurred (e.g., as in horses and donkeys).
   BULLET::::- At Chester Zoo in the United Kingdom, a cross between an African elephant (male) and an Asian elephant (female). The male calf was named Motty. It died of intestinal infection after twelve days.
  
  BULLET::::- Cagebird breeders sometimes breed hybrids between species of finch, such as goldfinch x canary. These birds are known as mules.
  
  BULLET::::- Gamebird hybrids, hybrids between gamebirds and domestic fowl, including chickens, guineafowl and peafowl, interfamilial hybrids.
  
  BULLET::::- Numerous macaw hybrids are also known.
   Though the hepadnaviruses have a very limited host range, with the potential cross-species transmission of hepadnavirus among primates and some families of birds, combined with recombinants, it might change the hepadnavirus host specificity. Interspecies recombination between hepadnaviruses from cross-species hosts would provide a large variation of virus genome, which will change the pathogenicity and transmissibility, and enlarge the host range; the first evidence of potential recombination between Chimpanzee HBV and human HBV genome was documented, recombination between human and non-human primate HBV strains, between GiHBV strains from different genera of gibbons, and between birds HBV strains from different avian sub-families were confirmed. All those experiments provided data allowing us to better understand the background, virus hosts, and zoonotic transmission of the non-human primate HBVs.
   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.
  
  Section::::Hybrid livestock.:Poultry.
   BULLET::::- "Cross-breed" animals : Genetic hybrids of wild and domestic parents. They may be forms intermediate between both parents, forms more similar to one parent than the other, or unique forms distinct from both parents. Hybrids can be intentionally bred for specific characteristics or can arise unintentionally as the result of contact with wild individuals.
  
  BULLET::::- "Feral" animals : Domesticates that have returned to a wild state. As such, they experience relaxed artificial selection induced by the captive environment paired with intensified natural selection induced by the wild habitat.
   Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often sterile, preventing gene flow between the species. Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, horses have 64 chromosomes, and mules or hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during 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.
   BULLET::::- Cheta was a chimpanzee of undetermined sex born about 1937 trained by George Emerson, stated to be the current chimpanzee under contract by Metro for the Tarzan films in March, 1943.
  
  BULLET::::- Unknown 3 was chimpanzee stated to have replaced the 1933-1943 Cheeta, cast in 1944 with a trainer from the St. Louis Zoo hired as handler for "Tarzan and the Amazons" (1945).
   Different chromosomes appear to have split at different times, suggesting that large-scale hybridization may have taken place over a period of as much as four million years leading up to the two emerging ("human" and "chimp") lineages as late as six million years ago.
  
  The similarity of the X chromosome in humans and chimpanzees might suggest hybridization taking place as late as four million years ago. This latter conclusion should be regarded as uncertain, with alternative proposals available to explain the apparent short divergence time in the X chromosome.
  
  Section::::In fiction.
  
  BULLET::::- "First Born" (TV series)
   When bred together, individuals of the same breed pass on these predictable traits to their offspring, and this abilityknown as "breeding true"is a requirement for a breed. Plant breeds are more commonly known as cultivars. The offspring produced as a result of breeding animals of one breed with other animals of another breed are known as crossbreeds or mixed breeds. Crosses between animal or plant variants above the level of breed/cultivar (i.e. between species, subspecies, botanical "variety", even different genera) are referred to as "hybrids".
  
  Section::::Breeding: selection by breeders.
   Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many types of animals, with the gray tree frog being a particularly well-studied example.
  