Marmoset chimeras

To my mind, one of the more fascinating biological phenomena is that of chimeras: individual animals which have two (or more) sets of genes. Chimeras have been discovered in a number of mammals, including human beings, but are rare.

Marmosets, on the other hand, are apparently all chimeras: they are almost always fraternal (non-identical) twins, and the twins swap placental cells in the womb. So each marmoset is born with two genetically distinct -- but related -- sets of cells.