How New Science Is Shaking up DNA Testing

December 30, 2017

 Photo Source: JSTOR

 

       In criminal as well as family court, DNA evidence holds the gold standard for determining the presence of a person at a crime scene or the paternity of children in question.   Every cell in our bodies contains DNA, and everyone gets half their DNA from their mother and the other half from their father.  DNA carries traits such as eye color and blood type that relate us to our parents, but at conception, DNA from both parents gets shuffled a little when creating the unique DNA that each person has.  Now, imagine a 52-year-old mother of three sons with worsening kidney problems who asks each of her sons to take a blood test to see if they might be a good match for a kidney donation.  When the results of the test come back, the doctors deliver a very different message than expected. They tell her not which of her sons could be a match but that apparently all of her three sons share DNA with the father, but only one of the three sons bears any genetic relation to her.   The doctors question whether her children belong to her at all.  She and her husband claimed she had three normal pregnancies, but authorities based on DNA cannot believe her.  

 

      How could a woman be the mother of sons with whom she does not share DNA?  The story sounds like science fiction or a bad dream, but in fact, it happened to Karen Keegan.  A group of talented doctors and scientists at Beth Israel Deaconess Medical Center in Boston, MA summarized how they solved the mystery in a report in the New England Journal of Medicine titled, “Disputed Maternity Leading to Identification of Tetragametic Chimerism.” (NEJM, 2002) Using very sophisticated testing of many different tissues in her body, the researchers determined that Mrs. Keegan’s body had two distinct sets of DNA.  In other words, she was a chimera. Greek mythology contains a fire-breathing creature called a chimera with the body and head of a lion, a goat’s head rising out of its back, the udders of a goat and a serpent for a tail. “Chimera” today in medical terms stands for an animal containing two or more distinct sets of DNA.  Initially, the researchers looked at many of Mrs. Keegan’s different tissue from her blood, skin, and hair follicles to look for a match and failed.  The breakthrough came when they tested her thyroid, and it contained distinctly different DNA that matched the DNA of her sons.  It turns out that Mrs. Keegan’s body contains cells of her fraternal twin that developed into distinct tissues in her body and as eggs in her ovaries.  Genetically, she is two people walking around as one person.

 

      Several years ago, Lydia Fairchild, an unemployed mother in Washington state encountered a similar situation.  (“She’s Her Own Twin,” ABC news 2016) When applying for public assistance, she and her kids needed to undergo genetic testing to prove their familial relationship.  The results definitively showed that she could not be related genetically to her children. The authorities suspected her of paid surrogacy and welfare fraud.  Her obstetrician vouched for every birth, but the courts considered DNA to be a higher standard than an eyewitness to the birth.  According to the article, a court officer attended a subsequent birth to test the mother and baby right away and again showed that the baby did not share DNA with the mother.  Fortunately, her lawyer heard of Mrs. Keegan’s case and asked for more testing, and Ms. Fairchild proved to be a chimera.  Her unborn twin sister appears to be the genetic mother of her children.  Another case of a chimera occurred again in Washington state; this time a man failed a paternity test because the DNA taken from his cheek did not match the child. (Independent, 2015) Only upon further testing did they learn that he as a chimera has genetically distinct DNA in his sperm from his cheek.   In a way, the DNA in his cheek genetically represents the uncle of his child.  Many times, the chimerism remains unseen and only detectable by DNA tests of different tissues.  In an article in The Daily Wire titled, “This Beautiful Model Is Her Own Twin Sister,” the author describes the musician, Taylor Muhl, from California diagnosed as a chimera in her mid-twenties strikingly shows a clear line running down the middle of her body with one twin on either side.  In some cases, people with two different colored eyes may be a chimera.

 

      Our DNA contains the traits that contribute to our eye color, skin tone, height, and more.  When we receive half of our DNA from each parent, some shuffling occurs, and our genetic code becomes unique to each individual.  The widespread use of genetic testing in criminal and family court relies on the DNA being unique to every person.  The accepted rule in court states one person has one unique genetic code. However, growing examples of human chimeras challenges the notion that one person consists of one set of DNA.  Chimeras form when the cells of fraternal twins combine after conception and develop as a single body.  The courts need to see the limitations of DNA evidence in the new light of chimeras.  Moreover, the concept of carrying the cells of a twin that did not survive separately after conception challenges our whole sense of self as we learn more about chimeras.

 

 

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