A chimera can refer to different things depending on the context. In Greek mythology, a chimera was a monstrous fire-breathing creature composed of different animal parts, usually depicted as a lion with the head of a goat protruding from its back and a tail that might end with a snakes head). In genetics, a chimera is a single organism composed of cells with more than one distinct genotype, meaning an individual derived from two or more zygotes). In humans, chimerism can occur when a person has cells from two different sources, such as when a fetus absorbs its twin or after a life-saving medical treatment like an organ transplant. People with this form of chimerism can appear entirely normal, so it is usually discovered only by accident. For this reason, it is not clear how common it is. Sometimes there are visible signs, such as differently colored eyes or patches of skin of different shades. When individuals are a mix of male and female cells, there can also be abnormalities in the reproductive organs.