enetics is confusing, and identity is confusing. It’s still hard to tell how much identity is due to genetics and how much is due to environment. But this is child’s play compared to the complex issues that increasingly arise with modern fertility treatments and assisted reproductive technologies. To say the least, we can not control genetics nearly as much as it confounds us and forces us to rethink laws and medical and court practices. 

In recent decades, it has become apparent that not even a DNA test is 100% accurate because there are rare genetic cases where people have more than one DNA line, a condition called chimerism, which has caused a lot of trouble in the courts, transplantation and transfusion medicine, fertility clinics, and forensics. And the crazy part is that chimerism is not in fact that uncommon but is rarely discovered because it rarely leads to observable abnormalities.

The additional cell line(s) may be acquired by transplantation, transfusion, or transfer of fetal cells into the mother, but in most cases, chimerism occurs spontaneously during embryogenesis. In vitro fertilization increases the likelihood of chimerism because it has an increased incidence of twins. Except for feto-maternal transfers, spontaneous human chimerism involves fraternal twins whose bodies have taken up cells of both genotypes from a single cell mass during embryogenesis.

About one-eighth of all conceptions and about one-eighth of live births are twins – most of them born alone, without a living twin. About one in eight people in the world is a twin born as an only child. Scary, right? 

You are actually not a father but an uncle

A Washington couple underwent fertility treatment and eventually had a healthy son in 2014. However, DNA tests showed that the father wasn’t related to his son. After ruling out his wife’s cheating, the man concluded that the fertility clinic must’ve mixed the sperm and impregnated his wife with another donor’s genetic set. The clinic checked their records thoroughly and couldn’t find any mistakes. 

The couple did further paternity tests, but the results kept showing that the father shared only 10% of his child’s DNA. Being very confused, they hired a lawyer, but he couldn’t help them much either. So he turned to Barry Starr’s blog Ask a Geneticist. Starr, the director of the Department of Genetics at Stanford University, suggested the couple have an analysis done by 23andMe, because the company provides more detailed genetic data on kinship and parentage – using hundreds of thousands of markers in the genome – than the dozen or so markers that paternity tests cover. 

When the report came in, it showed a 25% match, which meant the man was his son’s uncle. A 50% match would mean he was the father. It turned out that the DNA in the man’s sperm was 90% his DNA and 10% that of his unborn fraternal twin. A very strange way for nature to promote the existence of an unborn individual. This condition, where one twin dies and is “absorbed” by the other or by the mother or placenta, occurs in 20-30% of multiple pregnancies and is called vanishing twin syndrome. 

This means that the father had absorbed some of the cells of his twin in the womb and thus became a mixture or chimera of himself and his brother.

Chimeras aren’t so rare

The incidence of spontaneous human chimeras may be greatly underestimated, for such individuals are usually discovered only by accident. The first cases reported were found in blood banks when genotype tests found three or four versions of several genes instead of the expected one or two. And even in these cases, the disease often goes unnoticed unless the second cell line is present in 25% or more. 

But there can be drastic consequences if a hidden chimeric patient shows a dramatic reaction during surgery to a blood unit with a small proportion of a chimeric second lineage or the wrong cell type. In 2001, transfusion physician Willy Flegel and his colleagues at Ulm College Hospital reported a chimeric blood donor whose mixed blood showed altered RhD antibody profiles in some transfusion recipients. Their discovery led to a change in routine blood testing guidelines in German blood centers in 2002, and later in Austria, Switzerland, and the Netherlands, and triggered discussions about possible changes in routine blood testing in other European and North American countries.

The problem exists not only for chimeric patients who react badly to infusions with the wrong blood type but also for chimeric blood donors whose blood can endanger recipients. That’s why the Ulm group has applied a new molecular test for chimerism and DEL phenotypes to more than 8,400 blood donors found to be chimeric, but Flegel believes this isn’t the only case among the more than 100,000 donors a year at his institution. He explained: 

“Right now, we can’t give a more precise number. We’re still doing studies to find out how common this situation is. But even if it’s only 1 in 25,000, it represents a significant clinical problem.” 

Flegel added that thanks to the new molecular test for chimeras, chimeric patients can now continue to donate blood without dire consequences for recipients.

Thinking about the identity issues, it’s not hard to imagine a future battle for recognition of the special chimera identity and their rights.