A sheep–goat chimera (sometimes called a geep in popular media) is a chimera produced by combining the embryos of a goat and a sheep; the resulting animal has cells of both sheep and goat origin. A sheep-goat chimera should not be confused with a sheep-goat hybrid, which can result when a goat mates with a sheep.
The first sheep–goat chimeras were created by researchers at the Institute of Animal Physiology in Cambridge, England by combining sheep embryos with goat embryos. They reported their results in 1984. The successful chimeras were a mosaic of goat and sheep cells and tissues.
In a chimera, each set of cells (germ line) keeps its own species' identity instead of being intermediate in type between the parental species. Because the chimera contains cells from (at least) two genetically different embryos, and each of these arose by fertilization of an egg by a sperm cell, it has (at least) four genetic parents. In contrast, a hybrid has only two. Although the individual cells in interspecies chimeras are entirely of one of the component species, their behaviour is influenced by the environment in which they find themselves. The sheep-goat chimeras have yielded several demonstrations of this. The most obvious was that the woolly areas of their fleece,tufts of goat wool (angora type) grew intermingled with ordinary sheep wool, even though the goat breed used in the experiments did not exhibit any wool whatsoever.
Sheep–goat chimeras as a general rule may be assumed to be fertile, with the reservations that apply to chimeras generally (which again reflect that the parent embryos may have been of diffent sex, so that the animal, apart from being a chimera, may also be an intersex). But in accordance with the mosaic- (as opposed to hybrid-) nature of the interspecies chimera, any individual sperm- or egg cell it produces must be of either the pure sheep or the pure goat variety. Whether in fact viable germ cells of both species may be, or have been, produced within the same individual is unknown.
The term shoat is sometimes used for sheep–goat hybrids and chimeras, but that term more conventionally means a young piglet.