The young girl, naked and unconscious, was at the back of an abandoned jeep. She was severely beaten with bruises all over her body, with a dirty rag tied across her face to cover her eyes. When she regained consciousness, she started screaming and lashing out her hands, warding off anyone who approached her.

She was brought to the Child Protection Unit at the University of the Philippines-Philippine General Hospital for care and medical treatment. Eventually, the doctors were able to reconstruct what happened.

The girl was walking home from school when she felt someone grabbed her from the back. She struggled but was unable to free herself. Through the entire ordeal, she was not able to see her assailant’s face.

With the help of the trained child specialist, the girl agreed to provide samples from her body that contained DNA profiles, hers and two unknown males. The DNA samples were collected using a Sexual Assault Investigation Kit.

Close scrutiny of the DNA profiles revealed that the two males were related paternally – perhaps a father and a son or two brothers; the DNA analysis became more complex. Further DNA tests used a new battery of DNA markers, the so-called “Rapidly Mutating Y-Short Tandem Repeat Markers” (RM-YSTR) that are found only in males.

The RM-YSTR was used to generate the individual DNA profiles of each of the two males. During the criminal investigation, a knife known to be owned by the girl’s suitor was found inside the jeep. At trial, the court granted the petition for the conduct of DNA tests. In the end, the suitor and his 60-year old grandfather were consistent with the two male DNA profiles from the girl’s body. The two accused were sentenced to life imprisonment without the possibility of parole.

Notably, DNA evidence played a crucial role in the identification of the girl’s assailants. Given the need to continuously work towards increasing the capacity of DNA tests to distinguish two persons, and the high cost of DNA typing, the Department of Science and Technology-Philippine Council on Health Research and Development is supporting research at the University of the Philippines Diliman to formulate clear guidelines in the selection of specific DNA tests in any given investigation.

The research is being conducted under the Program on Forensics and Ethnicity at UP’s DNA Analysis Laboratory, Philippine Genome Center, Natural Sciences Research Institute.

The study aims to maximize the information derived from DNA tests, particularly in cases where insufficient or no evidence hinders the investigation and resolution of a case and denies justice to all.

In many instances, particularly in cases involving children, DNA may be the sole evidence that is available to identify the perpetrator of the crime. Currently, a set of 17 to 21 STR markers found on the Y-chromosome (called Y-STRs) is used in comparing male DNA evidence against male suspects. In a single-source DNA sample, the numbers of STR repeat blocks at these 17-21 markers comprise a Y-STR haplotype DNA profile.

Since the Y-chromosome is directly passed from father to son, all male descendants of a family line will have the same Y-chromosome and in effect, the same Y-STR profile. DNA testing takes advantage of this characteristic of Y-STRs in order to resolve paternal relationships, especially in cases of finding the real father of a male child and in identifying male cadavers and disaster victims using reference samples from male relatives.

The research will expand an earlier Philippine RM-YSTR database in order to assess the usefulness of 13 RM Y-STR markers in distinguishing between two Filipino men.

In order to do this, Filipino family trios composed of a man, his father and his son, or a man and his two sons were invited to participate. We typed the DNA samples donated at 21 conventional Y-STR markers and 13 RM Y-STR markers in order to compare the results of the two Y-STR panels. Continuing research will determine whether the use of RM Y-STRs will improve the capacity of Philippine investigators to differentiate between two paternally-related persons.

DNA, or deoxyribonucleic acid, serves as a genetic blueprint for the structure and function of all living organisms. The entire human DNA is packed into 22 pairs of chromosomes and a 23rd pair which is composed of two sex chromosome. Individuals that have two copies of the X chromosome (XX) are females while persons with one X chromosome and one Y chromosome (XY) are males.

The Y-chromosome, only found in males, is extensively used for criminal investigations, particularly in sexual assault cases. Given that more than 90 percent of assailants in sexual assault cases are males, the use of male-specific DNA tests simplifies the DNA analysis because it excludes the female victim. Forensic DNA typing of biological samples, such as semen, blood and other body fluids is done by extracting the DNA from these samples and examining the sequence at just several specific locations, called DNA markers.

The standard DNA markers used in forensics are called Short Tandem Repeat (STR) markers. These are composed of short combinations of A, G, C, and T (as short as 3 or 4 letters) that repeat in blocks throughout the DNA sequence. Forensic scientists look at the number of STR repeats in a particular site and compare this number with the number of repeats in the DNA of a persons suspected of committing a crime.

However, the conventional Y-STR marker panel has one problem: it cannot differentiate between males of the same paternal lineage. Because of the need to differentiate between two paternally related males, the Y chromosome was characterized further and new Y-STR DNA markers were identified. In 2011, a set of 13 Y-STR markers were found to mutate over 100-fold faster than the conventional Y-STR markers, and were referred to as “Rapidly Mutating Y-STR markers or RM YSTR.”

This panel of 13 RM Y-STRs was shown to very even between very close male relatives such as father and his son, between brothers and grandfather and grandson.

Written by:
Dr. Maria Corazon A. De Ungria, Lindsay Clare D.L. Carandag and Jazelyn M. Salvador
University of the Philippines Diliman

Published by:Department of Science and Technology-Science and Technology Information Institute (DOST-STII)