DNA evidence in paternity cases

Deoxyribonucleic acid or DNA testing is the most accurate form of testing to prove paternity or exclude paternity when the identity of the biological father is under dispute. DNA-based paternity testing has been requested to support claims for child support, inheritance, immigration and for peace in the family. More recently, DNA tests have been used to dispute false paternity claims that have already been decided in favor of the child’s mother prior to the submission of DNA evidence in US courts.

Traditionally, paternity is determined using evidence derived from the woman’s testimony, the defense of the alleged father where he claims sterility or that another man had had relations with the child’s mother, a presumption of legitimacy if the child was born within a valid marriage and physical resemblance between the putative father and the supposed child. In addition, either party may present the results of serological tests, e.g. ABO and MN blood typing, to support their respective claims.

In more recent years, the use of selected DNA markers has become the procedure of choice over blood typing because of the increased level of polymorphism and the less susceptibility of DNA molecules to degradation compared to proteins. The molecular stability of DNA is particularly important when dealing with environmentally challenged samples, e.g. exhumed bones and degraded tissues. Of the different methods of DNA analysis, STR (Short Tandem Repeat) typing is currently the most widely used because this method allows unambiguous scoring of DNA profiles, rapid processing and analysis.

DNA typing for paternity is done by first carefully extracting the DNA from the biological samples submitted by the alleged father, child with mother (paternity trio) or in the absence of the mother’s sample (paternity duo). The DNA pattern from the child is analyzed given those of his mother (if available) and alleged father. The DNA type contributed by the child’s real biological father should be observed in the alleged father. Then, the probability that the alleged father is the father of the child is calculated as a ratio between that of the alleged father and any random male in the population. Notably, testing without the mother’s DNA profile (motherless case) was found to be less informative and five times more prone to paternity inclusions when testing seven STR markers than when the maternal DNA profile is made available. The current DNA Laboratory set-up at UP-NSRI Diliman campus uses 16-20 STR analysis which includes the FBI-defined Combined DNA Identification System (CODIS) markers for DNA typing (www.dnaforensic.org). At the UP-NSRI DNA Laboratory, the lack of information brought about the absence of the mother’s DNA profile in motherless cases is minimized by increasing the number of DNA markers of the alleged father and child that are tested to 20 markers compared to the standard 16 markers for paternity trio cases.

A mismatch suggests that the alleged father is excluded as the biological father of the child. In some cases, mutation results in a false mismatch between real fathers and their children, hence the standard accepted in most laboratories is to require a minimum of two mismatches prior to excluding a man from potentially fathering the child. On the other hand, a match between the DNA profile of the alleged father and the child does not necessarily establish paternity, but may be due to chance matches between totally unrelated individuals. To estimate the likelihood of paternity over non-paternity, a Probability of Paternity (W) is calculated based on the DNA profile of the father, mother and child. The Supreme Court prescribed the minimum value of 99.9 percent for W.

Although the resolution of questioned paternity is normally a civil issue, it may also play an important role in criminal cases such as those involving rape when the victim also claims that the accused is the father of her child born out of the rape (criminal paternity). The first such case where DNA evidence was used was People v Paras (1999) where blood typing and DNA profiling results conclusively excluded the accused from being the father of the victim’s child. To the trial court, the date of the last incidence of rape stated by the victim is important since the child was born 10 months after the said date. According to the trial court, "these facts would be in violation of the rule of nature."

With the rapid development of DNA-based paternity testing, it is inevitable that DNA evidence will be used more and more to support or argue against paternity in the courts of law. Initially, the strength of paternity tests lies primarily in its power to exclude the wrong man. However, the rapid development of STR typing technology has also increased its power to identify real fathers, thus providing objective evidence for a fair and swift resolution of civil and criminal cases.

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Dr. Maria Corazon A. de Ungria currently heads the DNA Analysis Laboratory of the Natural Sciences Research Institute, University of the Philippines, Diliman campus (www.dnaforensic.org). This laboratory is currently promoting the development of forensic DNA technology in the country as well as the conduct of genetic studies of different Philippine populations.

Since 1998, the UP-NSRI DNA lab has been accepting requests for DNA typing in disputed parentage cases as well as assistance in criminal cases. Dr. De Ungria has received recognition from the scientific community as well as lay organizations for her various research endeavors, the results of which are aimed at putting science at the service of society.

Her awards include the Outstanding Young Scientist (OYS) award in 2003 from the National Academy of Science and Technology (NAST) and the UP Chancellor’s Gawad Hall of Fame for Best REPS in Research in 2005 from the University of the Philippines (Diliman). In 2005, she was the only woman who won one of The Outstanding Young Men (TOYM) award given by the Philippine Jaycees and TOYM Foundation as well as the Gerry Roxas Leadership Award from the Gerry Roxas Foundation.

Dr. De Ungria may be contacted via e-mail ([email protected]) or by calling the UP-NSRI DNA Analysis Laboratory (02925-2965, telefax).