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The news that researchers want to create human-animal chimeras has generated controversy recently, and may conjure up ideas about Frankenstein-ish experiments. But chimeras aren't always man-made-and there are a number of examples of human chimeras that already exist. A chimera is essentially a single organism that's made up of cells from two or more "individuals"-that is, it contains two sets of DNA, with the code to make two separate organisms.

These case studies heralded a new appreciation for the phenomenon of genetic chimerism-when an individual carries two or more genetically distinct cell lines in different parts of her body. Until the advent of techniques for blood typing and karyotyping cells, genetic chimeras where thought to be very rare. They only came to light when the phenotypes associated with the two distinct genomes were so discordant that the resulting individual was clearly exceptional, with patches of distinct skin coloration throughout the body, for example, or hermaphroditic genitals. In reality, genetic chimeras may be quite common, disguised in perfectly normal bodies harboring genetically distinct cell lineages.

Hank talks about chimeras, and why Venus the cat probably isn't one - but your mom might be!

A handful of scientists around the United States are trying to do something that some people find disturbing: make embryos that are part human, part animal. The researchers hope these embryos, known as chimeras, could eventually help save the lives of people with a wide range of diseases.

For the first time, biologists have succeeded in growing human stem cells in pig embryos, shifting from science fiction to the realm of the possible the idea of developing human organs in animals for later transplant.

The physical connection between mother and fetus is provided by the placenta, an organ, built of cells from both the mother and fetus, which serves as a conduit for the exchange of nutrients, gasses, and wastes. Cells may migrate through the placenta between the mother and the fetus, taking up residence in many organs of the body including the lung, thyroid, muscle, liver, heart, kidney and skin. These may have a broad range of impacts, from tissue repair and cancer prevention to sparking immune disorders.