A couple from Washington decided to undergo a paternity test after determining a mismatch of their son's blood type against their own. The father subsequently failed the said test. The man and his wife, who had help from a fertility clinic for their son's conception, then became anxious that the clinic may have made a mistake in the sperm donor during the procedures.

The clinic determined that no mistake was made as both son and father are white and no other white sperm donor came to the facility on the day of the son's conception.  The couple tried one other more precise paternity test, which also relied on swabbing the skin cells inside the father's cheek. This second test gave them the same result: paternity was negative.

The couple persevered to arrive at the truth and sought the help of Stanford University geneticist Barry Starr, who also manages the website Ask a Geneticist at the Tech Museum of Innovation in San Jose, California.  Dr. Starr suggested that the couple's case may actually point to possible far-reaching loopholes in standard paternity tests.

Upon his recommendation, father and son were administered a genetic ancestry test by 23andMe, which indicated that the supposed father is actually the boy's uncle.

"That was kind of a eureka moment." Dr. Starr realised at that point that the man's case was a case of human chimera.  This, in fact, is the first reported case of a paternity test affected by human chimera.

"Chimera reports are very rare but they are real."

Chimera occurs when the cells of a twin from a multiple pregnancy are absorbed by the surviving twin before birth. The birth in such cases becomes a single birth.

The confusion in the paternity test ensued because the man's saliva and sperm had different genes. The genetic ancestry test indicated that the father's sperm was a 10% genetic match to the infant.  The test also showed that the boy's father is the man's unborn twin.

Starr suggests that markers in paternity tests may need to be upgraded, similar to 23andMe's analysis, to accommodate more complicated or even unusual conditions, including chimerism.