Lies, Errors, or Chimerism? Unexpected DNA Test Results

Today’s DNA tests offer nearly 100% accurate results for identifying birth parents. Although these tests are almost always correct, There are some legitimate (but rare) scientific reasons they may return false results.

By false, I mean the results will identify a completely different person than the individual who tested. Human error, such as attaching the wrong person’s name to a test kit, may be to blame. Or, two people accidentally using the same cheek swab could cause false results. Occasionally, lab errors can render results invalid. However, usually those mistakes are caught early enough to allow companies to re-test users.

Double Helix Times Two

But what if a person’s saliva contains the DNA of more than one person? I’m not talking about giving your significant other a big sloppy kiss before spitting in the test tube. I mean, what if your body produces cells containing someone else’s DNA?

The test results would not be accurate. Assuming only one person is spitting in the tube or using the cheek swab, how can this happen?


When someone’s body contains two different sets of DNA, it’s called chimerism. Chimerism may be an acquired or congenital condition. A study published in the Journal of Assisted Reproduction and Genetics explains this phenomenon.1

Low levels of chimerism can be acquired via blood transfusions, or bone marrow or organ transplants, when cells from the donor become incorporated in the recipient’s body.

Tetragametic chimerism occurs when cells from dizygotic twin embryos fuse early in development leading to the birth of a singleton with two cell lines containing two different genomes.

Temporal (Acquired) Chimerism

Recipients of blood transfusions may carry bits of DNA from a blood donor for up to a year.2 “Spit kits” use saliva as a carrier for not only epithelial cheek cells, but also white blood cells. Although donated blood contains only trace amounts of white blood cells, there may still be enough present to interfere with DNA test results.3

Studies suggest organ transplant recipients may circulate traces of donor DNA for up to two years.4

Bone marrow recipients are discouraged from using home DNA test kits. This is because donor marrow will continue to produce cells containing the donor’s DNA.5

Tetragametic (Congenital) Chimeriam

Researchers have been aware of the phenomenon of human chimerism for decades. The first documented case was in 1953, when samples of two distinct blood types came from a single blood donor. 6

Usually, the condition shows no symptoms, but sometimes, chimerism is visibly apparent. Singer/model Taylor Muhl has an entire blog devoted to her experience with the condition. Since she was born, the left and right sides of her body have been different shades. One side is allergic to bug bites and costume jewelry, and the other side is not. She literally has the skin (and genetic makeup) of two different people.

Taylor Muhl
image source:

The Disappearing Fetus

Scientific American explains:

One way that chimeras can happen naturally in humans is that a fetus can absorb its twin. This can occur with fraternal twins, if one embryo dies very early in pregnancy, and some of its cells are “absorbed” by the other twin. The remaining fetus will have two sets of cells, its own original set, plus the one from its twin7

Vanishing Twin Syndrome was first discovered in the 1940s.8 It’s a long held-belief that many of these disappearing twins are absorbed by the mother or the placenta, and only sometimes by the other embryo.

Rare or Rarely-Diagnosed Condition?

To date, fewer than 100 people have been diagnose with chimerism. Due to the often invisible nature of the condition, it’s likely many chimeras will never know they carry two sets of DNA.

Experts suggest chimerism rates are much higher in known twins and triplets.9 Fertility treatments, which involve implanting several fertilized eggs at once, may also lead to higher rates of chimerism.

Animals, usually born with a litter of siblings, are even more prone to chimerism than humans.

Chimera Cat
image source:

Aunt or Mother?

There have been plenty of published case studies on chimerism, including the story of a mother named Karen Keegan who needed a kidney transplant. She had her sons tested as possible donors and they appeared not to be her biological children.10

It turns out, Karen had absorbed the fetal cells of a fraternal twin sister. Ms. Keegan’s eggs contained her sister’s DNA and not her own. The babies she carried were actually the biological children of her twin. Genetically, Karen’s sons were her nephews.

DNA Never Lies

In 2006, Washington state resident Lydia Fairchild had to go to court to prove she was the mother of her own children.

As a struggling single mother of two with a third child on the way, Fairchild, then 26 years old, decided to apply for government assistance. In order to qualify, Fairchild was required to undergo DNA testing to prove that she was the mother of children for whom she was claiming. When the test results came back, her world was shattered by an incredible revelation – she was not the mother of her two children.

Now facing criminal charges for fraud, Fairchild was ordered to have a court representative be present at the birth of her third child for an immediate DNA test, which revealed the same results.11

This was  before companies like AncestryDNA and 23andMe began offering home test kits. Had this test been performed today, Ms. Fairchild’s children would appear as her nieces and nephews. This is because, like in Ms. Keegan’s story, Ms. Fairchild’s eggs contained the DNA of her fraternal twin sister who she had absorbed in-utero.

Fairchild was certain a mistake must have been made, but she recalled a social worker saying to her, “Nope. DNA is 100 percent foolproof and it doesn’t lie.”12

If either Ms. Fairbanks or Ms. Keegan hadn’t undergone testing, they may never have known about their chimerism.

People Lie. DNA Lies.

Everybody Lies
Image Source: House M.D. Universal Media Studios

DNA saliva tests to determine maternity or paternity are nearly 100% accurate. When chimerism is suspected, a laboratory may analyze blood or hair to isolate the different sets of genetic code

If you’ve taken a DNA test and have a match listed as your biological father, odds are, that person is your father.

If he insists the results are wrong because he’s a chimera…

and he’s had a bone marrow transplant…

and a recent blood transfusion…

he’s probably lying.




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