Proving maternity

A short report published in Investigative Genetics recounts an instance where a DNA test gave results about maternity which were not as straightforward as they might seem.

A couple in China wanted to adopt a boy and were required to prove with a DNA test the child was not their offspring. An officially sanctioned DNA test, using STR markers akin to the CODIS markers used by the FBI and RCMP, showed the woman shared an allele with the boy at each of 46 autosomal STRs. That's what you'd expect if they were mother and son. Officially the test was interpreted as not excluding the possibility that the woman was the biological mother, which the woman denied.

As a result further tests were conducted with mitochondrial DNA and SNPs and showed with a high degree of certainty that the child was not her son.

What's the explanation? Read the short report here.