In humans, a chimera is a person who has cells from two different sources, resulting in two sets of DNA. This can occur when a mom is pregnant with twins, and one embryo dies, and the other one absorbs its deceased twins cells. Another mechanism is mosaicism, where there is a mutation of the genetic material in a cell, giving rise to a subset of cells that are different from the rest. Chimerism can also occur as a result of organ transplants, where cells from the transplanted organ can start growing in other parts of the body.

Most chimeras appear entirely normal, so it is usually discovered only by accident. However, 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.

It is important to note that the concept of a "human hermaphrodite" resulting from chimerism is largely a misconception. Most intersex individuals are not chimeras, and most human chimeras are not observed to have intersex traits.