What might produce a false positive on a paternity test?
DNA profiling is considered the most powerful and reliable way of paternity testing to determine the biological father-child relationship between two people.
However, some hidden details of this technology could cause false results.
A typical DNA test examines a small portion (about a millionth) of a person's genetic sequence that includes loci with unique repeating sequences to form genetic patterns. A so-called paternity index is originally derived from the number of matches between two sets of DNA as well as the likelihood of matches within a subject's ethnic group. The paternity index is then calculated using a simple probability equation to deliver a more understandable percentage, which is how paternity tests are normally reported. Therefore, the test does not show that the tested man is the only one who could be the biological father of the child. Rather, it shows how much the tested man has the genetic pattern that the child received from the biological father, hence the level of possibility that the tested man could be the biological father.
It is possible that other men in the population also have this pattern. Tests at a 99.0% probability of paternity identify the genetic pattern possessed, on average, by 1 in every one 100 men. Thus, a false-positive test occasionally occurs in which a DNA test shows that a man has a 99% or greater probability that he is the biological father of a child, when in fact he is not. He is merely one of the other people in the population who happen to have this genetic pattern. To have a perfectly accurate result, each element of the genetic sequence would need to be sequenced, which is currently limited by cost and technology constraints.
Some biological phenomena contribute to inaccurate results in DNA testing. Although almost all individuals have a single and distinct set of genes, rare individuals, known as chimeras, have at least two different sets of genes. In fact, several paternity tests have falsely shown that a mother was biologically unrelated to her children (1, 2). Another example of a genetic anomaly is mutation, that is, random alteration in genetic sequence. Simple genetic mutations that occur during mitosis can also cause a discrepancy in DNA testing, because the mutations in the child do not show up in the parent’s genetic sequence.
Even a perfect theoretical design cannot eliminate human error in practice. Complexity and multiple steps in the test increase the chance of human error. In a paternity-testing lab that processes thousands of samples per day, mislabeling, misinterpretation, and switched samples could happen but are not factored into the probabilities.
One thing to be learned from failed paternity tests is that human bodies are imperfect machines in which a range of biological alteration can take place.
1. Two Women Don't Match Their Kids' DNA. Abcnews.go.com. 2006-08-15. Retrieved 2010-04-03.
2. Parental Testing. Wikipedia.org.