The US man and his partner had a son using intrauterine insemination. When they later found that the child's blood type didn't match either of theirs, they carried out an at-home paternity test, which revealed the man was not the father of the boy. This was confirmed by a second paternity test.
Thinking that the fertility clinic must have mixed up the father's sperm with someone else's, the couple enlisted a lawyer and the help of geneticist Dr Barry Starr of Stanford University, California.
Dr Starr suggested that the couple use the company 23andMe for genetic ancestry testing. The report said the man was 25 percent related to the child, suggesting he was actually the child's uncle.
'That was kind of a eureka moment,' Dr Starr told Buzzfeed News, who said this was when they realised the man must be a chimera. 'Chimera reports are very rare, but they are real,' he added.
Dr Starr's team confirmed their suspicions by testing multiple tissues from the man. The buccal sample, which is used in standard paternity tests, showed he was not the genetic father of the child.
However, analysis of a semen sample from the man showed that approximately ten percent of its cells were genetic matches to the boy. DNA analysis of different tissue samples revealed multiple alleles of genes are present at the same loci – a phenomenon indicative of chimerism.
Chimerism can occur in the womb, when one twin dies early in pregnancy and the fetal tissue is absorbed by the other twin, the mother or the placenta. This is known as vanishing twin syndrome and is thought to occur in 20 to 30 percent of multiple pregnancies.
In this case, the cells the man had absorbed from his twin included germ cells, which develop into sperm, meaning his unborn twin's DNA was passed on to his child.
'This is certainly very unusual and interesting because it's a germline chimerism,' Professor Dieter Egli from Columbia University, New York, told Time. 'What is interesting is to explore in what way this affects the person's feeling of identity. These are questions that are more important as we start to use cell therapies and cell transplantations as well.'
Dr Starr suggests that paternity tests should include more genetic markers to detect chimerism. Although this is the first reported case of standard DNA testing excluding paternity due to chimerism, it may occur more frequently as the use of fertility treatments and incidence of multiple births increase, he says.
Dr Starr's team reported the findings last month at the International Symposium on Human Identification in Grapevine, Texas and the American Society for Human Genetics Annual Meeting in Baltimore, Maryland.