One Body, Two DNAs — Chimerism in Forensics

By Heena Chauhan, Budding Forensic Club

Chimerism is a biological condition where a single individual has two (or more) genetically distinct sets of DNA in their body. It’s named after the chimera, a mythological creature made up of parts from different animals.

The Chimera — myth and meaning

(In Greek mythology, the Chimera is a fearsome, fire-breathing monster known for its hybrid form and destructive nature. It is typically depicted as a lion with a goat's head protruding from its back and a snake's head for a tail. The Chimera was eventually defeated by the hero Bellerophon, who rode the winged horse Pegasus into battle. The term "chimera" has also come to refer to any creature with parts from different animals, or to describe something wildly imaginative or impossible.)

How Chimerism Happens?

There are several ways chimerism can occur:

1. Bone Marrow Transplant (most common in modern medicine)

   The patient’s bone marrow (which makes blood cells) is replaced with a donor’s marrow. Result → Donor DNA in blood, but the person’s original DNA in other tissues.

2. Twin-to-Twin Cell Transfer (Natural Chimerism)

   In the womb, fraternal twins can exchange cells through the placenta. Even if one twin doesn’t survive, the other can carry some of their cells for life.

3. Organ Transplant

   The transplanted organ contains donor DNA. These cells can be detected in blood or at the transplant site.

4. Microchimerism from Pregnancy

   During pregnancy, some fetal cells cross into the mother’s body (and vice versa) and can persist for decades.

Forensic Impact

In most people, all tissues have the same DNA. In chimeras, different tissues may have different DNA profiles. This can cause confusion in:

• DNA paternity/maternity tests
• Crime scene investigations
• Identification of remains

Key point: In cases involving bone marrow transplants or natural chimerism, a DNA sample from blood alone can be misleading — other tissues (hair, skin, reproductive tissue) may tell a different genetic story.

Quick comparison — sources and forensic signatures

Sources of chimerism and how they typically present in forensic testing

Source	Typical tissues affected	Forensic signature / risk
Bone marrow transplant	Blood (donor), other tissues (recipient)	Blood profile matches donor; tissue samples (hair/skin) match recipient — risk of false ID from blood-only tests
Twin-to-twin cell transfer	Variable — can include reproductive tissues	Paternity/maternity tests may mismatch depending on sampled tissue
Organ transplant	Transplanted organ and nearby cells	Transplant DNA can appear in forensic samples taken from donor organ or blood
Microchimerism (pregnancy)	Low-level presence in blood/organs	Usually subtle but may complicate sensitive assays

Real-life forensic chimerism cases

1. The Case of the Bone Marrow Transplant Murder Suspect

Background: In 2004, police in Alaska were investigating a violent assault. The attacker fled but left behind blood at the crime scene. Forensic testing produced a DNA match to a man already in the criminal database.

The twist: The man arrested had an airtight alibi — he was in prison hundreds of miles away when the crime happened. Blood DNA clearly matched him, yet he could not have been the attacker.

What they found: The arrested man had received a bone marrow transplant from his brother years earlier. His blood cells carried his brother’s DNA, so the crime-scene blood matched the donor brother — the real attacker.

Outcome: Further testing (hair follicles and skin) matched the arrested man himself; the brother was later arrested. This example is widely used in forensic training to show how donor-derived blood DNA can mislead investigations.

2. The Lydia Fairchild Case: The Mother Who “Wasn’t” the Mother

Background: In 2002 Lydia Fairchild (Washington State, USA) applied for government assistance and underwent DNA testing with her children. The lab reported that her DNA did not match her children — she was accused of fraud.

The shock: The father's DNA matched, but Lydia's blood-based DNA did not. Authorities suspected she was pretending to be the mother.

The discovery: A genetics expert suggested chimerism. Testing of different tissues revealed two genetic profiles: blood showed one profile (that didn't match the children), while cervical tissue and some skin samples showed another profile that did match the children. Lydia had absorbed cells from a twin in the womb. Her reproductive organs carried the twin's DNA, so her eggs were genetically of that twin.

Outcome: Lydia was cleared; the case is now a standard cautionary example in forensic genetics and family law.

So in short: bone marrow transplant chimerism stems from medical treatment (donor cells in blood), while natural chimerism (like Lydia’s) usually comes from twin cell fusion in the womb.

References

Further reading commonly cited in forensic genetics and chimerism literature:

• Butler, J. M. (2012). Forensic DNA Typing: Biology, Technology, and Genetics of STR Markers (2nd ed.). Academic Press.
• Saferstein, R. (2020). Criminalistics: An Introduction to Forensic Science (13th ed.). Pearson.
• Strachan, T., & Read, A. (2018). Human Molecular Genetics (5th ed.). Garland Science.
• van Dijk, M., Boersma, E. R., et al. (2008). "Microchimerism: Implications for Forensic DNA Analysis." International Journal of Legal Medicine, 122(1), 1–9.
• Reed, W., et al. (2004). "DNA Profiling After Bone Marrow Transplantation." Transfusion, 44(4), 853–861.
• Rink, B. D. (2008). "Chimerism in Humans: Implications for Forensic Science." Forensic Science International: Genetics, 2(1), 52–59.
• Lydia Fairchild case — widely reported in news and legal records (2002–2004)
• Bone marrow transplant mistaken suspect case (Alaska, 2004) — documented in forensic training materials

© 2025 Heena Chauhan — prepared for educational and informational use in forensic genetics discussions.