When Karen Keegan needed a kidney transplant, DNA tests on her sons showed she was not their biological mother. She was a tetragametic chimera: two fertilized eggs had fused in her mother’s womb, so her blood carried one genome and her reproductive organs another. This episode explains whole body chimerism, clues such as Blaschko’s lines that appear under certain UV light, and microchimerism, the cells mothers and fetuses exchange during pregnancy. It describes a 2010 case of a 40 year old man with a scleroderma-like illness whose female cells came from a vanished twin.

The hosts then turn to the animal kingdom: marmoset twins who share blood and can carry each other’s reproductive cells, male yellow crazy ants that need two unfused genomes, merged sponges and half colored budgerigars. Plants follow, from grafted fruit to variegated houseplants, along with a 2012 RNA and DNA hybrid virus found in Boiling Springs Lake in Lassen Volcanic National Park. The episode ends with lab chimeras and the ethical question of how many human cells make an animal human.

• About 95 percent of fraternal twin marmosets trade blood through fused placentas
• The 1600s Bizzarria fused the Florentine citron and the sour orange
• Beatrice Mintz made the first chimeric mice in the 1960s, followed by a goat and sheep geep in 1984
• Researchers made a sheep whose blood was 15 percent human in 2007
• A 2023 study reported a chimeric monkey with up to 92 percent donor derived cells in tested tissues