Hybrid animals are created when gametes (reproductive cells) from different species join to form a single embryo. A mule, for example, is the offspring of a female horse and a male donkey. Every cell in the body of hybrids contains genetic material from both parents.

Chimeras, named after creatures from Greek mythology, are created artificially by combining genetic material from different species into a single embryo. The adult animals that develop have different populations of cells that reflect different contributions from the species from which they were produced. Scientists have created the geep, for example, by combining genetic material from both a goat and a sheep.

Partially human hybrid embryos have been created by fusing human cells and animal eggs, and partially human chimeric embryos have been created by injecting human embryonic stem cells into animal embryos. Most scientists want to produce such embryos only for research, and oppose experiments that would allow human-animal chimeras to be brought to term.

The prospect of human-animal chimeras troubles many people and raises troubling questions about their moral and legal status. Would a human-animal chimera have human rights? Could it be patented and owned? What if it were 99.9% human and 0.1% chimpanzee? What of the reverse situation?