DNA is often considered the ultimate indicator of our identity – a foolproof way to determine our origins and how we connect to our parents and previous generations of our family. But in this excerpt from “Hidden guests: migrating cells and how the new science of microchimerism is redefining human identity” (Greystone Books, 2025), author and science journalist Lise Barnéoud explores an unusual case that reveals the limitations of DNA testingwhen a maternity test suggested that a woman was not the mother of the children she gave birth to.
“At first I kind of laughed … but they were serious. I could just see the seriousness on their faces,” Fairchild said. “DNA is 100% foolproof and it doesn’t lie,” a social worker told her. “So who are you?”
At first Fairchild was suspected of trying to defraud the welfare system by inventing children. The public prosecutor started an investigation and quickly confirmed that two children were indeed living with her. Could she have kidnapped them? Fairchild showed them photographs of herself pregnant. Her mother, the children’s father and the obstetrician all testified that she had given birth.
Could she be a surrogate mother who kept the children she had carried? After three hearings in court, Fairchild feared the worst. “Every day felt like it was going to be the last day I saw them,” she tearfully recounted. “I called every lawyer in the phone book. None of them believed me. It was my word against DNA. It was me against everybody.”
Fairchild was pregnant with her third child at the time, and the judge requested that both mother and child be tested immediately after birth. And the impossible happened: Fairchild’s third child, who had just come out of her womb, wasn’t her son either – genetically speaking.
Finally, a lawyer agreed to help her. Alan Tindell asked Fairchild about her life, her relationship with her siblings and her relationship with the father of her children. “Given her answers, I finally decided to believe her,” Tindell explained. He soon came across a scientific article describing Karen Keegan’s case and contacted the team in Boston to ask them to investigate Fairchild. They first tested Fairchild’s blood, but found only one cell type, just like they had for Karen Keegan. They moved on to cells from her skin, hair and cheek: still nothing.
We know too little about our own biology to have blind faith that DNA profiling will always reveal a person’s identity or parentage.
Lise Barnéoud, hidden guests
Until the day they performed a cervical smear. There they found cells with a different DNA, a DNA that matched both Fairchild’s child and her mother. They concluded that the other DNA must have come from a missing twin sister. Fairchild could finally breathe. But how would her story end without Karen Keegan?
The oft-taught equation “one individual, one genome” fails to capture the full complexity of reality. What seemed like a long-established and unshakable certainty, even to me, has turned out to be imperfect knowledge in need of revision. We know too little about our own biology to have blind faith that DNA profiling will always reveal a person’s identity or ancestry.
Our ultimate proof is far from foolproof. Nevertheless, it is very often used to establish relationships, prove or disprove paternity, assess applications for family reunification, or convict people otherwise presumed innocent. “The overarching assumption in such circumstances is that a sample that fails to confirm genetic kinship is indicative of fraud, regardless of other substantiations of legitimate kinship,” observes the British philosopher Margaret Shildrickone of the few researchers who examined the social and legal consequences of microchimerism.
Why is some scientific knowledge so quickly dressed up as infallible truth? Do we not dwell enough on our own ignorance? And why do some fields of knowledge remain frozen in skepticism, even when new discoveries should allow us to dispel our doubts? The sociology of science has its work cut out for it.
It is impossible to know how many Karen Keegans and Lydia Fairchilds there are. Most of the time, the existence of chimeric cells from missing twins goes unnoticed. If Keegan hadn’t needed a kidney transplant, if the Fairchilds hadn’t applied for welfare, they would never have known that their gametes were “occupied” by cells other than their own.
Their children or grandchildren may have eventually discovered that a branch in their family tree seemed to be missing, that they had somehow inherited genes that neither parent had.
Today, we know about a dozen cases of this phenomenon, known as germline chimerism: where chimeric cells are present in the tissue that forms eggs or sperm. One such case involved an American man who learned through a paternity test that he could not be the father of his child, who was conceived via assisted reproduction. He prepared to sue the clinic, believing he was the victim of a sperm mix-up, when a more precise test revealed he actually shared 25% of his DNA with the child. In other words, he was the child’s uncle, genetically speaking.
Further research showed that 10% of his sperm contained DNA from a missing twin brother. “One of the most impactful consequences of this case study is to point out that some traditional paternity tests that have resulted in negative outcomes (the tested parent was excluded as the biological parent) may have been wrong, because the alleged parent may have undiagnosed chimerism,” emphasize the researchers who chronicled his case.
Given the increasing use of these tests, it is likely that the paternity of other fathers has been wrongly disputed. It is precisely this scenario that is depicted in the French TV series “Nona et ses filles” broadcast in 2021. Nona, played by the 70-year-old actress Miou-Miou, is pregnant when a genetic test reveals that her boyfriend, André, cannot be the father of her child. They eventually learn that one of André’s testicles contains sperm from a missing twin brother. In the words of André, as he tries to analyze his situation, “So he’s my nephew…but he’s also my son.”
Edited and excerpted from the book Hidden guests: Migrating cells and how the new science of microchimerism is redefining human identity by Lise Barnéoud. Published by Greystone Books, 2025.
Hidden guests: Migrating cells and how the new science of microchimerism is redefining human identity
Part thought-provoking medical mystery – part groundbreaking science – “Hidden Guests” uncovers the astonishing phenomenon of microchimerism: the presence of foreign cells inside our own bodies. The incredible story of how these cells got there—and what they do when they arrive—could change everything we know about the immune system, lineage, and identity.
