Chimeras: When One Person Has Two DNA Profiles
DNA testing is widely regarded as the gold standard for confirming biological relationships. In most cases, the results are straightforward: a child inherits half their DNA from their mother and half from their father, and a paternity test simply checks whether those genetic markers match.
But human biology can occasionally produce surprises.
In rare situations, a person may carry more than one DNA profile within their body. When this happens, the interpretation of a paternity test becomes more complex and requires careful scientific investigation.
Two biological phenomena can lead to this situation: tetragametic (natural) chimerism, where two embryos fuse early in development, and transplant-induced (acquired) chimerism, which can occur after a bone marrow transplant.
Tetragametic (Natural) Chimerism
In mythology, a chimera is a creature composed of parts from different animals. In biology, a human chimera is a person who has two genetically distinct populations of cells in their body.
This can occur naturally when two fertilised eggs, which would normally develop into twins, fuse together very early in embryonic development. Instead of two individuals forming, a single person develops who carries cells from both embryos.
Because the two original embryos were genetically different, the resulting individual may carry two distinct DNA profiles.
Most people with chimerism never know they have it. The different cell populations are usually harmless and cause no health issues. However, the condition can become apparent during genetic testing.
For example, a person’s blood DNA may differ from the DNA found in their reproductive cells or other tissues. In a paternity test, this could potentially lead to unexpected results if the sample being tested does not represent the DNA that was passed on to the child.
Although these cases are extremely rare, they demonstrate that biology doesn’t always follow the simple inheritance patterns we expect.
Transplant-induced (acquired) chimerism
While natural chimerism is rare, another situation that can produce multiple DNA profiles is bone marrow transplantation.
Bone marrow transplants are used to treat certain cancers and blood disorders. During the procedure, a patient’s bone marrow, which produces blood cells, is replaced with stem cells from a donor.
Once the transplant is successful, the donor’s stem cells begin producing the patient’s blood cells. As a result, the patient’s blood DNA becomes identical to the donor’s DNA over time.
Because buccal swabs collect not only cheek lining cells but also white blood cells and other immune cells present in the oral environment, all of which carry the donor’s DNA after a successful transplant, the donor DNA may also appear in buccal samples. On top of that, bone marrow stem cells have the ability to migrate into other tissues, including the mouth, and establish themselves there permanently.
This can lead to mixed or unexpected DNA profiles during testing.
Case Study: When a Bone Marrow Transplant Changes the DNA Test
During the recent paternity testing of an adult child – let’s call him Daniel – our scientists encountered something that could easily have led to a false result in less experienced hands.
Daniel’s buccal swab produced a mixed DNA profile containing alleles from two different males in roughly equal proportions. At first glance, this might appear to be contamination. However, Daniel disclosed something that immediately changed the picture: he had undergone a bone marrow transplant approximately twelve months earlier.
This information was crucial.
Daniel’s buccal swab therefore contained DNA from two genuine cell populations: his own cells and those originating from his bone marrow donor.
To untangle the two profiles, we collected a blood sample from Daniel. This produced a single-source DNA profile, and comparison with the known maternal profile confirmed that the DNA belonged to the bone marrow donor, not to Daniel himself.
Our scientists then worked methodically to separate the profiles. By identifying the donor’s alleles from the blood sample and removing those alleles from the mixed buccal DNA profile, we were able to isolate the genetic markers belonging to Daniel.
The reconstructed profile was first confirmed against the known maternal DNA profile, ensuring the alleles were consistent with maternal inheritance. Only after this confirmation was the profile compared with the alleged father.
The result: paternity confirmed.
Without understanding the biological context, this case could easily have been misinterpreted. A laboratory that didn’t ask the right questions, or didn’t have the expertise to interpret an unusual DNA profile, might have reported the result as inconclusive, contaminated, or worse, drawn the wrong conclusion entirely.
Why Expert Interpretation Matters
Cases involving chimerism, or other unusual biological circumstances are uncommon, but they highlight an important point: DNA testing is not just about running a test – it is also about understanding the biology behind the result.
Experienced scientists know when something doesn’t look right and how to investigate further. That might involve collecting additional samples, analysing the data differently, or considering medical history that could influence the results.
Most paternity tests are straightforward. But when biology throws a curveball, careful interpretation ensures that the final answer is still accurate. If you’d like to discuss a paternity test or have questions about your results, our team at Identilab is here to help.
To learn more or speak with our team, call 1300 114 294 or email [email protected].
