Posted by & filed under Chimerism, Paternity Testing, Vibrant Gene.


You may have seen a CSI drama where a man is exonerated from a crime he actually committed because he has two different genomes (sets of DNA).  At the scene of the crime, he leaves traces of the alternate genome behind.  The lab tests the alternate genome, which doesn’t match the DNA in his blood.  He is erroneously declared innocent.  This might sound like science fiction, but this genetic condition is real and it silently impacts the lives of everyday people.  

What is chimerism?

The term chimera is derived from a Greek mythological creature that was composed of a goat, a lion, and a snake.  Although this monster never existed beyond our collective imagination, the health condition is quite real.  Chimerism is a condition whereby a person has not one but two complete genomes (sets of DNA) in their body.  One genome is found in one region or organ(s), while the other genome can be predominant in other organs or tissues.  So a DNA test result would be entirely different depending on where the sample was originally from (blood, saliva, fingernail clippings or hair, etc).  As you can imagine, this condition can lead to confusing results when the person seeks medical care or relationship testing!


One young chimeric father I know reported being sent to a hospital after a car accident.  When the hospital staff discovered he has two different, incompatible blood types flowing through his veins, they asked if he had received a bone marrow transplant.  Hospitals are usually ill-equipped to identify and treat chimeric individuals.



One mother’s story

Perhaps the most famous example of a human chimera is Lydia Fairchild.  Lydia applied for child support for her two children after separating from her husband in 2002.  At the time, she was pregnant with their third child.  She was required to prove a biological relationship to the kids, so she submitted DNA samples for routine DNA relationship testing.  Her husband was confirmed as their biological father, but the test indicated she was not the mother.


How could this be possible?  Lydia conceived both of her kids naturally, carried them to term, gave birth, and was being told she was not their mother.  Without knowledge of her condition, Lydia was accused of fraud. She nearly lost her two children to child protective services.


Lydia was court ordered to have a witness present at the birth of her third child to take a DNA sample immediately following the birth.  Again, the third child’s DNA test excluded her from being the mother.  Thankfully, a geneticist thought to test her cervical cells’ DNA, and indeed that DNA matched her son’s genome.  In essence, Lydia had delivered a child from her twin, thereby becoming both the child’s mother and biological aunt. Take a moment to let that sink in.


What causes chimerism? 

There are many types of chimerism, and various root causes.  For the type of chimerism mentioned in this blogpost (also known as ‘tetragametic’ chimerism), the most common cause is a twin pregnancy that naturally reduces to a single baby.  This is referred to “vanishing twin syndrome” because one embryo doesn’t survive and the embryo’s cells are absorbed by its twin and the mother.  The remaining embryo incorporates the disappearing (“ghost”) twin’s cells into various tissues and develops to a healthy baby.


Back to Lydia Fairchild, and her story.  What most likely happened is Lydia absorbed cells from her fraternal (non-identical) twin while still in utero.  Her twin did not survive to term and all that remained of the embryo were the cells that Lydia retained in her ovaries and other organs.  Lydia’s blood had her dominant genome. So when social services took a blood sample for routine maternity DNA testing, it did not match her child’s genome.



Lydia unknowingly passed on her second genome to her baby.  In doing so, Lydia became the child’s mother and simultaneously, the biological aunt.    


An even more common form of chimerism is called microchimerism.  Microchimerism is defined by a person harboring a small percentage of cells that originate from another individual.  Frequently, this condition arises during pregnancy when a mother and the fetus exchange cells across the placenta.  Microchimerism has been detected in women who have never had a full-term pregnancy, but may have been unknowingly pregnant and miscarried.  A transient form of microchimerism can also occur after a blood transfusion or organ transplantation.  The baby’s stem cells may be acting to help protect the mother from her own autoimmune condition. For reasons not fully understood, many mothers report relief from rheumatoid arthritis during pregnancy. There is reason to believe that microchimerism may be the underlying explanation.



How does a person learn that they have chimerism? 

Most chimeras are believed to be asymptomatic and completely unaware of their condition.  Medical literature has identified some common symptoms including having different colored eyes, patches of different skin tones, different colored sections of hair, and, sometimes, disorders of sexual differentiation (formerly called hermaphroditism).  Some chimeras have autoimmune diseases because the body recognizes the twin’s genome as a foreign substance.  This does not suggest that anyone who has different colored eyes or skin, or an autoimmune condition has chimerism.



How common is chimerism?  The answer might surprise you…

To understand how frequent chimerism might be, we first need to understand some things about twin pregnancies.  A surprising number of single babies were originally twin pregnancies, where the twin did not survive to term.  An estimated one in eight (or 12%) babies born had a twin that disappeared, and left no evidence of their existence.


Unfortunately, we don’t have enough scientific evidence to make an accurate prediction for the rate of chimerism in humans.  Currently, there is no public-wide screening program in the US, nor is there any real need for it since most chimeras are healthy folk.  We do know that, through the popularity of assisted reproductive technologies (such as IVF), twinning is on the rise, and we can anticipate chimerism to become more common.  Microchimerism is believed to be the most common form of chimerism, with an estimated 20-90% of women harboring cells from prior pregnancies.  The rate of microchimerism in men is less well known but assumed to be the same as women.



What increases the likelihood of chimerism?

Anything that increases the likelihood of a twin pregnancy increases the chances of a person being an undiagnosed chimera.  If your parents utilized assisted reproductive technologies, such as IVF or used fertility medications, your chances of having been a twin pregnancy are more likely.  Left-handedness has been observed more frequently in twins, leading some experts to believe that a left-handed individual may be more likely to have had a twin in utero.



What can I do if I suspect I have chimerism?

Chimeric individuals usually live healthy, normal lives–until something goes wrong involving DNA testing.  If you have had a parental DNA test performed, and the result was (falsely) negative, chimerism may be an explanation.

Contact us at for additional information.  We have a proven track record for successfully identifying chimeric individuals through cutting-edged DNA testing, and advocating for parental rights.




2 Responses to “Giving Birth to Your Sibling’s Baby: The Intriguing Condition of Chimerism”

  1. Karen

    My husband has two different colored eyes. When younger he was always told he had blood type AB when giving blood and medical procedures. Recently previous to s surgery he was told that his blood type was A.
    His mom was given blood transfusions while pregnant with him. Any connection?


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