Two Beginnings, One Body

Note: This story is fictional but draws on real events; in particular, the case of Karen Keegan, described by Yu et al. (2002).

It was an ordinary Tuesday when we got the results.

“You are not the mother.”

I laughed. Not because it was funny, but because the alternative was just too ridiculous to take seriously.

“The test is wrong,” I said. “Do it again.”

“We already did.”

The geneticist sat opposite me, choosing her words carefully.

“The results are consistent with your husband being the father,” she continued. “But we haven’t found the genetic markers we’d expect to see from you.”

I stared at her, then looked at my husband. He’d been there for everything: the first ultrasound, the strange cravings, the swollen ankles, the sleepless nights. He’d held my hand through labour and watched our daughter enter the world, her first cry erupting from her tiny lungs and filling the room. I had carried her for nine months. I knew I was her mother.

My first instinct was to look for a mistake. Had the samples been mixed up? Had someone put the wrong label on the tube? Could there have been contamination, or was there an error in the laboratory? Surely, there had to be an explanation.

The geneticist explained that the laboratory would first review the sample identification, records and quality controls. The original findings needed to be checked before anyone could draw conclusions. Depending on the circumstances, new samples might be collected and tested.

I clung to the possibility that something ‘ordinary’ had gone wrong.

But the discrepancy remained.

I wanted to understand what the test had actually found. The geneticist explained that a parentage test compared particular regions of DNA, looking at genetic markers that a child would ordinarily inherit from their biological mother and father. A child receives roughly half their nuclear DNA from each parent, and the markers examined should be consistent with that inheritance.

My husband’s genetic profile was consistent with fatherhood. Mine wasn’t.

If my daughter had inherited half her DNA from me, where was it?

I’d always thought of DNA as like a fingerprint – something fixed and consistent throughout the body. A sample of blood should identify the same person as a swab from inside the cheek. I’d never had any reason to question that assumption.

But the human body can be more complicated than we imagine.

Chimerism is a rare condition in which a person carries two or more genetically distinct populations of cells. The name originates from the Greek mythology, Chimera — a creature made up of different animals. In humans, the reality is less monstrous but no less extraordinary: a single person can carry two or more genetically distinct populations of cells.

I struggled to understand how someone could live an entire life without knowing. Was it possible that my own body held something like this?

One form, called tetragametic chimerism, can occur when two eggs are fertilised by two different sperm. Ordinarily, each fertilised egg develops into a separate embryo, potentially becoming fraternal twins. Very rarely, the two early embryos fuse, and their cells combine to develop into a single person.

The person may carry two distinct genetic lineages, each originating from a different fertilised egg. Four gametes, two beginnings, one body.

Someone with chimerism may grow and develop normally, with no outward signs that anything is unusual. The two genetic lineages can be distributed differently throughout the body. A blood sample might predominantly reveal one, while other tissues contain evidence of another.

The possibility was extraordinary. But the doctors needed evidence.

They arranged for DNA from additional tissues to be examined. By comparing genetic profiles from different sources, they could investigate whether my blood told the same story as the rest of my body.

It didn’t.

The genetic profile found in my blood did not fully match those from other tissues. Further analysis was needed to establish whether the differences reflected two distinct genetic lineages rather than technical problems.  

Eventually, the tests confirmed it. I carried two genetically distinct cell populations, with profiles consistent with two separate fertilised eggs having contributed to my body.

I had tetragametic chimerism.

The words seemed almost impossible to reconcile with the life I’d lived. All my life, I’d assumed I had one genetic identity. Yet my body carried evidence of two beginnings, and I’d grown up without the slightest idea.

At first, the discovery felt like an answer. It explained why the DNA in my blood had failed to show the expected genetic relationship with my daughter. But it also raised a question far more frightening than anything I’d considered before. 

What did this mean for my daughter?

If my body contained two genetic lineages, which one had contributed to my daughter? The discovery of chimerism could explain why the original test had failed to identify the expected maternal relationship. But it could not, by itself, answer every question about the DNA I had passed on. 

Further investigation would be needed to determine whether my daughter had inherited genetic material from my second lineage. For the first time, though, the results offered an explanation for something that had seemed impossible.

The test hadn’t necessarily been wrong. It had analysed the DNA in the sample it was given, but couldn’t capture the full complexity of my genetic makeup. 

I was still the woman who had carried my daughter, given birth to her and brought her home. None of that had changed. But now I understood something I had never imagined possible: the DNA we’d tested wasn’t the entire story.

Bibliography

Semikhodskii A, Makarova T, Sutyagina D. Paternity pseudo-exclusion caused by tetragametic chimerism in a gestational surrogacy case. European Journal of Medical Genetics. 2023 Aug 1;66(8):104799. https://doi.org/10.1016/j.ejmg.2023.104799

Yu N, Kruskall MS, Yunis JJ, Knoll JH, Uhl L, Alosco S, Ohashi M, Clavijo O, Husain Z, Yunis EJ, Yunis JJ. Disputed maternity leading to identification of tetragametic chimerism. New England Journal of Medicine. 2002 May 16;346(20):1545-52. https://doi.org/10.1056/NEJMoa013452

Yu Q, Li Q, Gao S, Su Y, Deng Z. Congenital tetragametic blood chimerism explains a case of questionable paternity. Journal of Forensic Sciences. 2011 Sep;56(5):1346-8.  https://doi.org/10.1111/j.1556-4029.2011.01794.x