Imagine this scenario: you’re sitting at home staring at the TV when there’s a knock at the door. As soon as you answer, police officers rush in and take you into custody. The crime? Murder. And, they’ve got you by the proverbial short hairs because your DNA is a match for that found at the crime scene. The only problem is that you didn’t do it because you might be a chimera.

In Greek mythology, a chimera was a creature that was a combination of a lion, a goat, and a dragon who, of course, breathed fire. In humans, animals, and plants, a chimera has two entirely different sets of DNA.

All About DNA

DNA, or deoxyribonucleic acid, was first discovered in the late 1860s by Swiss chemist Friedrich Miescher. American biochemists Phoebus Levene and Erwin Chargaff determined the chemical composition of DNA, but the molecule’s structure remained elusive.

Beginning in the summer of 1951, and continuing over the next 18 months, visiting American James Watson and British Francis Crick single-mindedly pursued the structure of DNA while working together at Cambridge University’s Cavendish Laboratory. If you want to read a spine-tingling real-life who-dun-it, pick up Watson’s 1968 book, The Double Helix.

On February 28, 1953, Watson and Crick deduced that DNA was comprised of two twisting polynucleotide chains held together by the nucleobases cytosine (C), guanine (G), adenine (A), and thymine (T), forming a double helix. The sequence of the base pairs contained all the information needed for the growth, functioning, and reproduction of all known organisms including some viruses.

For their work, Watson, Crick, and King’s College London scientist Maurice Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine. Sadly, left out of the prize was King’s College X-ray crystallographer Rosalind Franklin whose images were key to understanding the structure of DNA. Franklin died in April 1958, possibly due to exposure to X-ray radiation.

It wasn’t until 1990 that an attempt to sequence the entire human genome began. The Human Genome Project was a worldwide consortium of scientists primarily from the U.S., UK, France, Germany, Japan, and China. It had an initial budget of $3 billion and a timeline of 15 years but was completed two years early in April 2003.

There were still slightly less than 400 gaps that remained in the sequence, but on March 31, 2022, the Telomere-to-Telomere (T2T) consortium announced that they had filled in the remaining gaps and produced the first truly complete human genome sequence. The Human Genome Project has made its research publicly available, enabling the sharing of biomedical data worldwide.

How Chimeras are Created

Chimeras can be created in any of the following ways.

Bone marrow transplants — Up until the late 1960s, a diagnosis of blood cancer was a death sentence. Then, doctors had success trying a novel treatment first proposed in 1957 by Dr. E. Donnall Thomas whereby radiation and chemotherapy were used to destroy a patient’s bone marrow, then the patient was infused with healthy bone marrow cells from a donor. For his discovery, Dr. Thomas received the 1990 Nobel Prize in Physiology or Medicine.

In 2015, a Reno, Nevada man who had leukemia received a bone marrow transplant from a donor in Germany. The Reno man worked in the Washoe County Sheriff’s Department crime lab. Before his bone marrow transplant, also known as a stem cell transplant, a colleague suggested he take samples of his blood.

Four years later, and cured of his disease, his colleagues took DNA samples from his lips, cheeks, and semen. The lip and cheek swabs showed a mixture of the man’s DNA and that of his donor, but incredibly, 100 percent of his seminal DNA was that of his donor. That means that in the unlikely event the criminalist committed a rape, any DNA recovered would match that of his donor and not him.

This is exactly what happened in a 2004 case in Alaska when investigators got a DNA match in a rape case. The only problem was that the supposed perpetrator had been in prison at the time of the attack. Investigators were stumped until they learned that the man had received a bone marrow transplant from his brother, and the brother was convicted of the crime.

Siblings in the womb — An additional way to acquire the DNA of another is to be one part of either fraternal or identical twins. Fraternal twins result from the fertilization of two separate eggs with two different sperm while identical twins result from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two.

During pregnancy, fraternal twins often swap DNA and one twin can absorb the other twin. Such was the case for singer Taylor Muhl, who discovered in 2009 why half her torso, both front, and back, was covered in two completely different skin colors. Chimeras frequently have one eye that is one color and the other eye that is another color, and they can also have two completely different blood types.

In 2002, when a woman needed a kidney transplant, doctors tested her immediate family for a tissue match. Imagine the woman’s surprise when she was told that she couldn’t be the mother of two of her three sons. Taking a closer look, doctors discovered that the woman was a chimera whose blood cells contained one type of DNA and whose ovaries contained an entirely different type.

The DNA of identical twins is almost 100 percent the same which made determining which of two twin brothers committed a rape in Boston in 2004 almost impossible.  The case languished until 2014 when Belgium firm Eurofins Scientific succeeded in identifying the DNA of one twin from that of the other due to each twin acquiring unique genetic mutations while still in the womb. While a judge ultimately disallowed the Eurofins’ test results, prosecutors were still able to make their case against the guilty twin.

A souvenir of pregnancy — During the 1990s, scientists discovered that women can retain some of the DNA of their babies in a process called microchimerism. An easy way to test for this is to look for any Y chromosomes in women who have given birth to boys. Women carry two X chromosomes while men carry an X and a Y chromosome.

A 2015 study in the Netherlands of 26 women who had died shortly after giving birth to sons showed Y chromosomes in every tissue including kidneys, livers, spleens, lungs, hearts, and brains. Male DNA was even found in the brain of a 94-year-old woman who had last given birth decades before.

Diagnosing Chimerism

Many of us might be chimeras and not even know it. Typical ways in which chimerism is discovered include:

• Submitting a DNA sample to one of the DNA database companies such as 23andMe, AncestryDNA, MyHeritage, Family Tree DNA, and Living DNA, and having it come back with unexpected results
• Compatibility testing for organ or tissue transplants
• A baby having two different skin tones, a condition known as hyperpigmentation, or two different eye colors
• A single person having two different blood types
• Some children who are born intersexed

Chimerism is common in both the plant and animal world. Many male tortoiseshell cats are chimeras, while recent research has shown that most marmosets are chimeras and share DNA with their fraternal twins. Chimerism in the plant world includes dual-colored roses and African violets.

Today, scientists don’t know how many people have chimerism as the vast majority of cases go undiagnosed. If you suspect you have chimerism based on the list we’ve included above, you may consider consulting a genetic specialist, such as a genetic counselor. A chimerism genetic counselor can evaluate your medical history, interpret your standard parental DNA test results, design a chimerism-specific testing strategy, and analyze the results.

While chimerism rarely has a negative impact on an individual’s health, a diagnosis has the potential to change their life by delivering understanding into previously unexplained events.