Frequently Asked Questions

What is DNA and how it is important in crime investigation?
DNA stands for deoxyribonucleic acid. DNA is a hereditary molecule that has genetic code for the structure and function of a living organism. DNA is found in all living organisms. DNA is a long molecule and its structure is like a twisted ladder. The steps of the ladder are made up of four “bases” Adenine (A), Thymine (T), Cytosine (C) and Guanine (G) whereas the sides of the DNA ladder is made of sugars and phosphate molecules. The genetic information lies in the sequence (orderly arrangement) of nucleotide base along the length of DNA molecule. An adult human body contains approximately 100 trillion cells. All the cells in our body have DNA with the exception of red blood cells. DNA is same in all the cells of an individual. DNA is packed in the form of chromosomes. Humans have 23 pairs of chromosomes. DNA sequence is 99.7% same in all the humans. Only 0.3% variable region in DNA sequence is unique in every individual. A set of markers in this variable region is studied by forensic DNA scientists to reveal the unique genetic identity of human individual using biological evidence. Biological evidence, which contains DNA, is a type of physical evidence that can connect an offender/victim to a crime scene if the DNA profile of a suspect matches the DNA profile of crime scene evidence. Biological evidence includes blood, semen, saliva, skin and tissue cells deposited on crime scene or obtained from the suspects or victims in a crime.

Is DNA of every individual unique?
Yes, DNA of every individual is unique except for identical twins who share 100% of their DNA profile. Other siblings share more than 50% of their DNA profile. A parent and child share 50% of their DNA profile. Unrelated individuals have several dissimilarities in the DNA profile.

How DNA evidence is preserved for DNA analysis?
If the biological evidence is properly dried and it is prevented from heat, chemicals, bacteria, fungus, moisture and sunlight; it will not be degraded and will remain suitable for DNA analysis.
How long DNA evidence can survive?
DNA evidence can survive indefinitely if it is dried and protected from heat, moisture, microbes and chemicals.
In which cases Blood can play its role as evidence?
Blood may be significant in the cases of murder (homicide), attempt of murder and physical assault.
What forms of Blood evidence can be present on a crime scene?
Blood may be present in liquid form or as dried stains on the body and clothing of victim/suspect; on the weapons/tools used in physical assault; on articles used for self-defense; or it may be found deposited on crime scene.
What are the collection methods for each form of blood evidence?
Liquid blood is collected in sterile syringe or container. Dried blood can be collected using a moist, sterile cotton swab. Thick dry stains of blood may be scratched with a sterile blade and collected in paper and a druggist fold is made. Wet blood stains can be collected using a dry, sterile cotton swab. Small blood stained articles may be collected, dried and packed as a whole.
If victim’s blood is present on suspect’s body/ clothing/ crime scene; is it helpful in solving a crime?
Yes, this type of biological evidence can be helpful in linking the victim/suspect to the crime.
If victim’s blood is present on an article retrieved from suspect’s home, car or office; is it helpful in solving a crime?
Yes, this type of biological evidence can be helpful in linking the suspect to the crime.
If suspect’s blood is present on victim’s body/ clothing/ crime scene; is it helpful in solving a crime? How to identify whether the blood belongs to victim or the suspect?
Yes, this type of biological evidence can be helpful in linking the suspect to the crime. Any blood stains foreign to victim may be identified by the blood spatter pattern.
What reference sample of deceased victim is collected in homicide cases?
Usually, cardiac blood sample is collected as a reference sample during autopsy (post-mortem). If autopsy samples are not available, then the DNA identity of victim may be determined by DNA analyses of personal belongings of victim e.g. tooth brush, razor, clothing etc. Alternatively, the reference samples from victim’s parents would be required.
In which cases, semen acts as an evidence?
Semen acts as probative evidence in cases of rape, sexual assault and sodomy.
Where semen evidence can be found on crime scene?
Semen may be found in vagina, rectum, oral cavity, body surface, hair, or clothing of the victim, crime scene surfaces and condom used during rape.
What type of evidence is collected from the rape victim?
Following evidence items should be collected from the rape victim:
  • Vaginal, rectal and oral swabs of the victim (for the detection of semen and DNA analysis).

  • Swabs from bite marks and licked/kissed areas (for the detection of saliva and DNA analysis).

  • Any foreign hair on victim’s body (for the detection of foreign DNA). Victim’s clothing (for the detection of semen and DNA analysis).

  • Fingernail scrapings in case of self-defense and scratching of the suspect’s body.

What type of evidence is collected from suspect of the rape?
If the rape suspect is arrested within few hours of rape, his penile swabs must be taken for the detection of victim’s DNA along with reference buccal swabs.
Is the reference sample of victim’s husband also required for forensic DNA analysis?
Reference buccal swabs of victim’s husband/sexual partner are required for elimination purpose
Is semen required as a reference sample from suspect in rape cases?
No, semen is not required as a reference sample from suspect in rape cases. All body cells contain similar DNA; therefore, buccal swabs will be collected as reference sample.
For how long semen remains detectable in a vaginal swab of the victim?
Generally, semen may be detectable on vaginal swab collected within 72 hours of sexual intercourse.
Who collects the vaginal/rectal swabs of the victim?
An authorized and qualified medico-legal doctor will perform medical examination and collect vaginal/rectal swabs. The swabs must be completely dried, packed and sealed in paper envelops.
What is a reference sample and why it is required? What type of reference sample is taken for DNA analysis in PFSA?
A reference sample is required for determination of a suspect’s, victim’s or witness’ DNA identity so that it could be compared with DNA profiles obtained from crime scene evidence. This comparison between crime scene and reference sample may result in a match, mismatch or inconclusive results. In case of a match, a link between reference and crime scene sample is established that can be proved in the court of law.
What is touch DNA? Where it can be present at crime scene?
Touch DNA is the DNA obtained from skin cells of an individual that are transferred to an article on intimate contact. Touch DNA may be found on firearms, tools, eating and drinking utensils, jewelry items, mobile phones and contact areas of clothing of the individual.
In which type of cases saliva may be useful as biological evidence and where it is found on crime scene?
Saliva may be useful in any type of case where the oral contact of an individual is suspected. Evidence items that may contain saliva include cigarette butts, drinking bottles and cups, chewing gums, eaten food items, bite marks and licked articles.
Which body parts may be useful in DNA analysis for dead body identification in mass disaster, accident and unidentified bodies?
Long bones, teeth and nails may be useful for dead body identification.
What reference samples may be required for missing person and unidentified body identification?
The reference sample required for the determination of DNA identity of a missing person or unidentified body includes the personal belongings of victim e.g. tooth brush, razor etc. Alternatively, the reference samples from victim’s parents would be required.
Can victim change clothes, wash clothes and body parts or urinate after the rape/sodomy incident before evidence collection?
No, victim must not change clothes, wash clothes and body parts before evidence collection. She/he should urinate only if there is an urgent need, however, the genital area should not be washed after urination. Only dry tissue papers can be used to wipe the genital area and this tissue is also preserved and collected as evidence.
Why should blood and semen stains on clothing be dried before packing? Which type of bag is used for the packing of biological evidence?
Blood and semen stains on clothing must be dried before packing so that the DNA evidence is well preserved. The dried biological evidence should be sealed in a brown paper bag and labeled properly.
What information must be written on the evidence packing and what documentation is required?
Description of item, case number, item number, date and time of collection, initials of the collector and victim’s name should be written on the packing. Date and initial should be written in a way that part of them is on the evidence tape and the rest is on envelope. Photographs of the evidence items should be taken before collection and after packing using size scales.
What is contamination and how it is prevented?
Contamination is the addition/mixing of foreign DNA in biological evidence. The source of this contamination may be the person(s) handling the evidence, the environment, or other evidence items. Following measures may be taken to avoid contamination:
  • Employing careful collection technique and using personal protective equipment i.e. gloves, face mask and coat.

  • Changing gloves between handling of different evidence item.

  • Having minimal contact with the evidence items i.e. by using sterile forceps.

  • Avoiding drinking, eating, sneezing and talking near the biological evidence.

  •  Preventing mixing of different evidence items in same packing.

Is biological evidence hazardous for the person collecting it? What preventive measure should be taken while handling biological evidence?
All biological evidence must be considered potentially hazardous because hepatitis and AIDS viruses, pathogenic bacteria and human parasites may be present in biological material. Personal protective equipment (gloves, face mask, coat, goggles) must be worn during evidence handling.