A chimera is a creature from Greek mythology. It's made up of parts of several different animals. But when the term applies to humans, it's very real and very, very weird.
We're told DNA never lies. But there have been cases of mothers not sharing their DNA with their children. One such woman was Lydia Fairchild. Fairchild had two children and was pregnant with the third, all by the same father. But she had fallen on hard times and applied for welfare. The US state of Washington wanted to make sure the father was, indeed, the father of the children. They took DNA from Fairchild and her partner, Jamie Townsend. But the results showed she was not the children's mother.
And so began one of the strangest court cases in the world. Fairchild was suspected of welfare fraud and it seemed nothing she said, or did, could prove otherwise. Welfare officials began to wonder if she had kidnapped the children.
Fairchild was in deep trouble until fate stepped in with a strange co-incidence. In Massachusetts, Karen Keegan needed a kidney transplant and her family was tested to see if any of them were a match.
According to the DNA test, she was not the mother of her children. Keegan's doctors were soon asking the same questions as the Washington welfare department.
They checked to make sure the babies had not been swapped at birth or kidnapped, bought or adopted. They were puzzled. Keegan's story came to the attention of a doctor of transfusion medicine, Lynne Uhl. The doctor tested Keegan again and this time all three of her boys were tested. The youngest was shown to have inherited her DNA but the two oldest had not.
Some of Uhl's colleagues suggested Keegan could be a chimera. Uhl took DNA tests from all over Keegan's body, hunting for the part that would have different DNA. Eventually they found it, in her thyroid tissue.
No one was hunting for Fairchild's DNA, though. But luck was still on her side. She was about to give birth to her third child and the court sent someone to witness the occasion and a subsequent immediate DNA test. The third child did not share her DNA either.
By chance the prosecutor read about Keegan's case in a science journal and thought the same thing could apply to Fairchild. Eventually they found it did.
No one is really sure how this happened. But the theory is that both women started out as fraternal twins in the womb - different eggs fertilised at the same time. The eggs fused to become one baby but some of one egg's DNA remained in the second egg, as a sort of record of its identity.
Well, that's the theory anyway.