People with Double DNA: An Intro to Human Chimeras
Are You a Chimera?
"Chimera" is pronounced ki-meer-uh.
What is a chimera? In biology a chimera is a lifeform with two DNA profiles. For example, a human chimera's hair sample might not match their saliva sample. It's crazy but true!
The word "chimera" comes from ancient mythology. It refers to an imaginary creature comprised of various animal parts, e.g., a lioness combined with a snake and a goat. Scientists didn't realize until very recently that people and other animals, along with plants, can express more than one set of DNA. In fact, you might be a chimera!
Some people are rather obviously possible chimeras. For instance, if you have mostly coffee-brown skin plus patches of whitish skin, you might be expressing two different sets of DNA. Other human chimeras stand out for having hair that doesn't match the rest in color or texture. Intersexed people may also be chimeras. However, many human chimeras have no external signs of their unusual condition. Their second set of DNA might only operate in their kidneys, for example.
This article about human chimeras was inspired by the Discovery Channel special entitled "I Am My Own Twin."
How Do People Become Chimeras?
Chimeras are created when four parent cells (or two fertilized eggs) become fused. Transplants can also create chimerism since they introduce foreign DNA.
Although human chimeras are likely as old as our species, DNA wasn't identified until 1952 with Rosalind Franklin's X-ray crystallography. A human chimera was announced the following year: In 1953 in the British Medical Journal published the first study of a human chimera. The woman in the study had two distinct blood types. Her chimerism came from her twin brother, who was still alive. She had absorbed some of his DNA while in the womb.
Being a twin seems to be the most common way of becoming a chimera. The other twin might be unknown since twin pregnancies sometimes end up as single births.
Despite the publication from 1953, most medical doctors were unaware of chimerism just a decade ago. The Discovery episode "I Am My Own Twin" documents two cases of chimerism in the US that have helped raise awareness.
"I Am My Own Twin"
In the first case, a Washington woman named Lydia Fairchild applied for welfare assistance. She was required to prove her relationship to her children. Oddly enough, her children's DNA tests suggested that she wasn't their mother. However, it did confirm that their father was their father! Prejudice worked against Fairchild; people figured she'd been caught attempting fraud. She couldn't get legal representation because lawyers regarded DNA "evidence" as irrefutable.
Fortunately, a "more respectable" woman in Boston was experiencing a related situation. Karen Keegan was a 52-year-old teacher. When she needed a kidney transplant, her three sons were tested for DNA compatibility. The tests suggested that she lacked a maternal relationship with two of her three sons.
While Fairchild was fighting off the state prosecutor, Keegan was in a position to pursue the DNA mystery. Her doctor pored over medical journals and networked with colleagues. She then surmised that Keegan was a chimera and began testing.
First, doctors took DNA samples of Keegan's hair and skin. When these samples proved fruitless, they moved on to sample her internal organs. That's when they found a different set of DNA. Genetically, two of her sons appeared to be descendents of her husband and a vanished twin.
Luckily for Fairchild, Keegan's experience was documented in a medical journal. When word reached Washington, Fairchild was finally able to secure legal representation, welfare, and custody of her children.
Implications of Chimerism
As Fairchild's judge observed, chimerism calls into question the validity of DNA tests. Positive matches remain unchallenged but all "negatives" are now suspect. The phenomena of chimerism calls into question many paternity suits and other court cases. In a small percentage of couples, it might also help explain infertility.
Learn More About Chimeras
- Chimeras: Transplants & Failed Paternity Tests
Genetic testing isn't perfect, as revealed by the case of Lydia Fairchild.
- Mosaicism - The Genetics of Chimeras
- Animal-Human Hybrids Spark Controversy
Mice with human brain cells and humans with pig tissue are only two real-life examples of chimeras. This article from National Geographic explores the ethics of creating human-animal hybrids.
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