Do 2 animals have to be in the same genus to interbreed or does it vary widely? I'm not too knowlegable on cross breeding to make a good analysis of how it works.
Technically the definition of species is one in which the members cannot interbreed with members of a different species. So organisms within the same genus should not be able to interbreed.
This has some wiggle room depending on certain definitions. For instance two organisms might be from different species if they do not interbreed in the wild, for instance if they occupy completely distinct geographical areas, or if they do not recognise each other as mates. However in an artificial setting or through IVF it might be possible to produce offspring, but the species definition is intact if it doesnt happen naturally. The species definition is also still intact if organisms can reproduce but produce infertile offspring.
Problems arise because the species definition is somewhat abitrary, as many organisms are on what could be deemed a spectrum of speciation whereby some populations reproduce between each other but do so rarely, or when they do hybrid depression (where the offspring is less fit than either parent), means there is a selective advantage to not interbreeding. So that in reality animals often have to be studied on a case by case basis.
Following on from Phil's post. The vast majority of animal hybrids are the offspring of parental species assigned to the same genus. Exceptions I can think of would include hybrids that can occur between sheep (genus Ovis) and goats (genus Capra). In the Galliforme birds (chickens, pheasant, guineafowl etc) I believe there are some birds assigned to different families that are known to hybridise to produce viable offspring(though I don't know if the offspring are ever fertile themselves).
Do bear in mind though that if the species concept is problematic then higher order taxonomic units are basically fairly arbitrary. It may well be that the degree of genetic differentiation between families in one group of animals is similar to that among genera (or even species) in another - there is no common scale here.
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