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A Man's Unborn Twin Fathered His Son Thanks To Genetic Chimerism


Tom Hale

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist


The case became the subject of a study published in the Journal of Assisted Reproduction and Genetics in 2017. Image credit: Leptospira/

This is the story of a failed paternity test, genetic chimerism, and a man whose child was unknowingly fathered by his own unborn twin.

After a series of fertility treatments, a baby boy was born to an anonymous couple from Washington in June 2014. The birth seemed relatively unremarkable – until routine blood testing highlighted a discrepancy between the blood types of the parents and the baby. Confused by these initial findings, the man was asked to take follow-up paternity tests. It was revealed he was not the father of the baby.


Some scientific investigations uncovered that was not a case of infidelity, nor a blunder at the fertility clinic. Remarkably, the man was unknowingly a chimera – an individual composed of cells with more than one set of DNA – and the child’s paternal genes were from the DNA of the man’s unborn twin that was lost in early pregnancy.

The Washington case became the subject of a study published in the Journal of Assisted Reproduction and Genetics in 2017. The researchers explain that genetic testing revealed the man and the child only shared 25 percent of their DNA – the same amount shared by an uncle with a niece or nephew. A full biological parent, on the other hand, should share 50 percent of their DNA with their child. This was the point where things started to click into place and researchers started to speculate whether a case of chimerism was muddling with the results.

Tetragametic chimerism is a form of congenital chimerism. This condition occurs when two separate eggs are fertilized by two sperm. These two separate structures go on to “meld” at the blastocyst or zygote stage, resulting in the development of a single organism with intermingled cell lines and effectively two sets of DNA in different parts of their body. Chimerism has been found in humans before, but it’s thought to be exceptionally rare. In 2009, an American singer discovered she had tetragametic chimerism after investigating a rare birthmark that ran down the center of her body. Estimates vary, but most sources suggest there have only been around 100 cases documented in humans. 

This case in Washington, however, was even more unusual because the cells from the man’s unborn twin were incorporated into his reproductive cells, giving rise to sperm of two distinct cell lines. In other words, some of his sperm cells were "his", while others contained the genetic material of his unborn twin. When they carried out the paternity test, they used the father’s cheek cells that only displayed his set of genes, not the twin genes found in the sperm cells, hence the unusual result. 


The man went on to conceive a second son with the same partner. This time around, the child was born with his sperm cells, as shown by paternity tests that found the man and second child shared 50 percent of their DNA. This means that the two children were not technically full or half-siblings, but shared an estimated 37.5 percent of their genes (full siblings share around 50 percent of their DNA, half-siblings around 25 percent).

A similar case study was reported in the New England Journal of Medicine back in 2002. Karen Keegan and her apparent adult sons underwent DNA tests prior to a kidney transplant she needed, revealing that she was not their biological mother. It was eventually discovered that Keegan had tetragametic chimerism.

“Even geneticists are blown away by this,” Barry Starr, a geneticist at Stanford University who was involved in the Washington case, told BuzzFeed News in 2015. 

Starr suspects that there might be many more cases of chimerism out there, but most remain hidden because they don’t show any outward physical signs. With the advent of easily accessible DNA tests, however, this might soon change. 


“Chimerism wasn’t even really something we thought a lot about until DNA testing came of age,” he added. “We are going to find more.”


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