How Can 1 Person Have 2 Different Sets of DNA?

Image by Elias Sch. from Pixabay

A human with two different sets of DNA is called a chimera, and it’s more common than you might think. Most chimeras don’t even know they have this strange phenomenon going on inside them.

You could be a chimera, and so could I.

As we go along, take note of the interesting tidbits you could twist into a plot to add conflict.

Without any help from the scientific community, the process of becoming a chimera occurs naturally. Numerous books and movies explore chimerism using a killer who’s had a bone marrow transplant or blood transfusion. But are these characters based in fact?

Let’s take a look and find out.

The tissue inside our bones is called bone marrow, and it’s responsible for making white blood cells, red blood cells, and platelets. When someone has a bone marrow transplant, doctors use chemotherapy or radiation to destroy all the recipient’s diseased bone marrow. The donor’s healthy marrow is then introduced and continues to produce blood cells with the donor’s DNA, thereby transforming the recipient into a chimera.

In some cases, all of the blood cells in a person who received a bone marrow transplant will match the DNA of their donor. But in other cases, the recipient may have a mix of both their own blood cells and donor cells. A blood transfusion will also temporarily give a person cells from someone else, but in a bone marrow transplant, the new blood cells are permanent, according to the Tech Museum of Innovation in San Jose, California.

What if we’ve never had a transplant?

Doesn’t matter. There are other ways to become a chimera.

Early on in pregnancy a mother can be carrying fraternal twins and one of the embryos might die in utero. The surviving embryo may absorb cells from the deceased twin. When the baby is born, s/he can have two sets of DNA. Since twin loss occurs in 21-30% of multiple-fetus pregnancies, think of how many chimeras could be walking around. Are the story wheels spinning yet?

It can also happen with a normal pregnancy.

In the 1990s, scientists discovered that a pregnant woman may retain some DNA from her baby, if fetal cells happen to migrate into her bloodstream and travel to different organs. The New York Times referred to this as a “pregnancy souvenir”— but it’s more scientifically known as “microchimerism.”

A 2015 study suggests this happens in almost ALL pregnancies (you read that right), at least temporarily. The researchers tested tissue samples from the kidneys, livers, spleens, lungs, hearts, and brains of 26 women who died while pregnant or within one month of giving birth. The study found fetal cells in all of the women’s tissues. The researchers were able to tell the fetus cells from the mothers by searching for Y chromosomes (only found in males). The deceased mothers were all carrying sons.

Writers: Don’t take the obvious road. Think victims instead of killers.

  • What if a human brain washed up on the beach?
  • What if the Medical Examiner wrongly assumed the victim was male due to the Y chromosomes?

This is one way to use research to our advantage.

  • What if the brain contained animal and human DNA?

Remember, we’re thinking victim, not killer, which puts a different spin on it.

According to Live Science, fetal cells may stay in a woman’s body for years. In a 2012 study, researchers analyzed the brains of 59 deceased women ages 32 to 101. A shocking 63 percent had traces of male DNA from fetal cells in their brains. The oldest woman died at 94 years old, suggesting that these cells can sometimes last a lifetime.

The blood-brain barrier is the body’s defense system to block many drugs and germs in the bloodstream from entering the brain, but doctors have found this barrier becomes more permeable during pregnancy, which may explain how these fetal cells migrated into the brains of their mothers.

  • What if a serial killer only targeted people with chimerism because s/he viewed them as freaks of nature?
  • How might the killer find potential victims?

If you said the medical field, you’re not thinking outside the box.

Interestingly enough, 26 of the 59 women had no signs of brain disorders while alive. The other 33 were diagnosed with Alzheimer’s disease. The researchers found that women with Alzheimer’s were less likely to have male DNA in their brains than women without the disease.

Previous work on microchimerism suggested fetal cells might protect against breast cancer and aid tissue repair in the mothers, but could increase the risk of colon cancer. Microchimerism can also incite various autoimmune diseases. Autoimmune diseases occur when a person’s body is mistakenly attacked by its own immune system.

Past research suggested Alzheimer’s is more common in women who had a high number of pregnancies than in childless women. One of the limitations of this research is that the number of brains studied was relatively small. Other researchers involved with microchimerism want to explore what effects a mother’s cells might have in her offspring’s development and health.

Imagine all the different scenarios? Parts of your writer brain must be on fire by now. No? Then check this out …  

Are you a chimera? 

You may never know. Unless you wind up in a similar situation to a woman named Karen Keegan. In 2002, her story became a report in the New England Journal of Medicine after doctors told her that she wasn’t the biological mother of her children.

Imagine? Think of all the ways this one conversation could implode an MC’s life.

  • Maybe the woman’s marriage broke up and the only reason her and her husband reunited was because she said she gave birth to his child while he was stationed overseas.

Turns out, the DNA in Karen Keegan’s bloodstream didn’t match the DNA in her ovaries. The doctors later determined she’d most likely absorbed a fraternal twin in utero.

How’s the ol’ writer brain feeling now?


This entry was posted in #amwriting, #writerslife, #writetip, Writing and tagged , , , by Sue Coletta. Bookmark the permalink.

About Sue Coletta

Sue Coletta is an award-winning crime writer and an active member of Mystery Writers of America, Sisters in Crime, and International Thriller Writers. Feedspot and named her Murder Blog as “Best 100 Crime Blogs on the Net.” She also blogs at the Kill Zone, Story Empire, and Writers Helping Writers. Sue lives with her husband in the Lakes Region of New Hampshire. Her backlist includes psychological thrillers, the Mayhem Series (books 1-3) and Grafton County Series, and true crime/narrative nonfiction. Now, she exclusively writes eco-thrillers, Mayhem Series (books 4-8 and continuing). Sue's appeared on the Emmy award-winning true crime series, Storm of Suspicion, and three episodes of A Time to Kill on Investigation Discovery. Learn more about Sue and her books at

23 thoughts on “How Can 1 Person Have 2 Different Sets of DNA?

  1. Sue, amazing information. Thanks for your great research.

    I can only imagine what poor Karen Keegan said to the docs. “If I’m not their mother, why the hell did I carry them around in my belly for nine months?”

    • Fabulous idea, Alan! Your writer brain is in perfect working order. Bravo!

      Thank you for the link. Bookmarked that puppy. 🙂

  2. Sue, this is fascinating. I can see it now: the Greek mythology graduate student who receives a blood transfusion from a killer and begins a search to destroy all chimeras, not realizing he himself is one.

    Brrr. Gives me chills just thinking about it.

  3. Fascinating, Sue. You have such a talent for writing about your research.

    My mind immediately goes to a victim who has both human and animal DNA…

  4. Wow! Interesting post. I had not heard of this before. Another great topic to bring up with my sis. She has a PhD in Experimental Psychology and this is just the stuff she studies and teaches: human development, the brain, and all the possible abnormalities and deviations. Never thought to ask her about this. Thanks for sharing.

    • Ooh, I love Experimental Psychology. Thank your sister for me, Cecilia. Folks in her profession provide fascinating articles that send me down one rabbit hole after another.

  5. Uhb, speaking of animal and human DNA from one person, I wonder what they would find if they could test the DNA of a skinwalker–a shape-shifting creature who is human but can change into an animal.

    The shapeshifting skinwalker is most present in the tales and lore of the Navajos. There are those who claim that the creature can also be found in and around the vicinity of the so-called Skinwalker Ranch in northeastern Utah, very close to Fort Duschesne, home of the Northern Ute tribe. And very close to the now-closed White Rocks Indian School, a federal boarding Indian school where my family lived during WWII while my Dad took the place of an Indian Service employee who was in the service. Brrrrrr.

  6. My story needs this type of error but only with the Y chromosome.
    There is a teardrop at the murder scene. Cursory DNA analysis suggests it belongs to the victim because it is so similar (except for the sex)—her killer is a brother. I need a way for analysts to overlook the male factor. Does anyone know how/if that could happen?

    • Nancy, there are two terrific groups on Facebook where you’ll have access to experts in all investigative fields. I suggest you ask this question in one of the groups. The two groups are: Writers’ Detective and Trauma Fiction

      Best of luck with your story!!!

  7. You dig up the most interesting stuff, Sue! I’ve never heard of a chimera or chimerism. You’ve opened up a whole new plot portal 🙂

  8. Pingback: How Can 1 Person Have 2 Different Sets of DNA? | Loleta Abi Author & Book Blogger

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