Chimeras and Paternity Testing
A Washington couple has some shocking answers about the true paternity of their child. After the results of a paternity test last year concluded that the man believing to be the father was not, the couple has been on a wild search for the truth. More extensive DNA tests revealed what happened. The truth was that the man had been a twin in the womb, but absorbed the genes of his unborn sibling (and the rest of him!), thus giving him a rather confusing genetic profile.
In June of 2014, the Washington couple were overjoyed by the birth of a healthy baby boy, whom they conceived using IVF. However, their baby bliss was soon interrupted when they learned that he did not share the same blood type with either his mother or his father. The blood test requested prompted the couple to utilize a home paternity test to get answers. But the shocking results of the test only left them with more questions; it indicated that the man was not the boy’s father.
The couple feared that the fertility clinic they used for IVF had used the wrong semen, the lab denied the possibility since the father was documented to be the only while male to provide a sample on the day their son was conceived. So, desperate to find answers, they hired an attorney and registered for a hopefully more accurate legal paternity test. However, the results of the new tests yielded the same troubling results.
Searching for Answers
The couple then turned to a Stanford University geneticist, Barry Starr, for help. He said, “You can imagine the parents were pretty upset. They thought the clinic had used the wrong sperm.” At Starr’s suggestion, they conducted a direct-to-consumer ancestry test, which interestingly enough showed that there was indeed a familial relationship detected between the two, except as uncle and nephew rather than father and son. It was at this time when Starr had “kind of a eureka moment” and began to suspect that the father may actually be a chimera.
The ancestry test led to Starr and his colleagues at the Department of Genetics at Stanford University School of Medicine to conduct a thorough case study with extensive DNA tests used to get to the bottom of the chimera conundrum.
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The case study thoroughly detailed their findings and confirmed his suspicions in a published report that said, “This condition was suspected in this case after a pregnancy conceived with intrauterine insemination revealed an exclusion of paternity, but ancestry testing revealed an avuncular relationship between the father and child. Multiple tissues from the alleged father were tested. The buccal sample from the alleged father was consistent with a single source DNA profile that excluded the child in question. Analysis was consistent with a single source DNA profile that excluded the child in question. Analysis of a semen sample from the alleged father revealed the presence of multiple sets of alleles at several loci, consistent with chimerism, and included the child in question. Additional tissues showed a similar multiallelic profile.”
What is Chimerism?
Chimerism occurs in utero when a fetus absorbs the genes of what would have been a twin. This medical anomaly occurs very early in pregnancy. The absorbed DNA remains part of the genetic makeup of the fetus during gestation and throughout life. Many view chimerism to be incredibly rare, but it actually occurs more often than many of us are ever aware of as it goes largely undiscovered as it is difficult to identify. There are currently only around 100 verified cases on record, but a study from the American Journal of Medical Genetics suggests that it may occur in up to 8% of pregnancies with multiple embryos.
How Does it Effect Paternity Testing?
The failed paternity test in question here is new territory for scientists involved in the case study, though. “To our knowledge, this is the first reported case in which paternity was initially excluded by standard DNA testing methods and later included as the result of the analysis of different tissues. This case of chimerism yielding a false exclusion is thought to be unusual,” the report concluded.
24% of the 400,000 paternity tests cases that are conducted each year are reported to prove that the men tested are not the biological fathers of the children in question. It is possible and even probable that instances of chimerism are behind some of the these cases. Starr suggests that rates of chimerism may even increase as more and more families turn to fertility treatments to conceive and as pregnancies resulting in multiple embryos being implanted. He says, “Chimerism wasn’t even really something we thought a lot about until DNA testing came of age. We are going to find more.”