In 1953, the first documented case of chimera was discovered. When “Mrs. McK” donated blood in northern England, her blood was screened and shown to contain two different blood types. Her case was truly an enigma. It was a scientific fact that every person has one, distinct blood type so how could she possibly have two?
Robert Race and Ruth Sanger at the Medical Research Council Blood Group Unit in London recalled a case of twin cows that contained each other’s blood types due their blood vessels crossing during pregnancy. They therefore reasoned that “Mrs. McK” must have been a twin. “Mrs. McK” confirmed that she had a twin bother who had died at the age of three. By this reasoning, she was conclusively recognized as the first human chimera.
When Lydia Fairchild, a 26- year old mother of two, applied for public assistance from the state of Washington in 2002, she was required to submit a DNA sample to prove relatedness between her and her children. The DNA test confirmed her partner’s paternity but excluded her as the children’s biological mother. As the state officials questions her, she adamantly insisted that she was the children’s biological mother. She substantiated her claim using birth certificates and photographs, all proving that she had birthed these two children. Lydia happened to be pregnant at the time of the investigation. After giving birth to her third child, she and the child were immediately given DNA tests. These tests, again, excluded Lydia as the biological mother.
After being accused of being the surrogate mother of her third child, Lydia sought legal assistance. Alan Tindell took on Lydia’s case and soon uncovered the medical case of Karen Keegan, a case similar to Lydia’s own. When, at the age of 52, Karen Keegan was in need of a kidney transplant, her family was tested to see if any of them was a match. To everyone’s shock, Karen’s two children were not genetically related to her. To solve this case, DNA samples were taken from all over Karen’s body and were tested against her sons’. The DNA from her previously removed thyroid came back as a match to her sons’ DNA. Interestingly enough, her thyroid DNA was genetically different than the DNA throughout the rest of her body. After Tindell brought Karen’s case to attention, Lydia underwent similar testing that proved that she had some matching DNA in other parts of her body that had not been previously tested, ultimately proving that she was the biological mother of her three children.
Lydia Fairchild and Karen Keegan are both chimeras. Although Lydia did not have a living twin, she had a twin during gestation. The two fetuses fused into a single fetus, ultimately developing into one person with two distinct cell types. The absorbing of one fetus into another, as in the cases of Lydia and Karen, is called Vanishing Fetus Syndrome.
When chimerism results from inheritance, it is referred to as microchimerism. Microchimerism results from the free exchange of fluids between a fetus and its mother. Meaning, traces of fetal cells can migrate throughout a mother’s body, infiltrating her heart, brain and blood. Similarly, maternal cells migrate towards the fetus and infiltrate its DNA. Microchimerism is frequently found in women who have had abortions. It has also been observed in the finding of male DNA in women who have only had daughters, or have had no children at all. Doctors are taking advantage of the DNA exchange between mothers and their
fetuses by using simple blood draws as DNA tests, rather than the more invasive tests such as amniocentesis and chorionic villus sampling. The interesting finding about microchimerism is that it is contained within all of us. As a fetus, each of us freely exchanged our DNA with our mother’s. These cells may not aggregate in a fetus and are therefore less detectable than they would be in the mother.
Since it is human nature to pride ourselves on our individuality, knowing that our DNA is not uniquely our own becomes a troubling fact for many. There does lie comfort in the fact that although every person’s genetic makeup contains DNA from her mother, and even from her siblings, the composition of that makeup is and always will be uniquely her own.
Sources:
http://www.the-scientist.com/?articles.view/articleNo/42476/title/From-Many–One/
http://www.thedailybeast.com/articles/2015/10/07/dna-at-the-fringes-twins-chimerism-and-synthetic-dna.html