People can leave traces of their DNA at a crime scene because it is inside every cell of their body. DNA can be extracted from blood, semen, saliva or hair roots left at a crime scene using a chemical process. Tiny amounts of DNA can sometimes be extracted from a single cell – such as cells shed from someone’s skin when they touch an object using new sensitive techniques (known as ‘low copy number’ DNA).
Police can also collect biological samples from suspects, usually by scraping some cells from inside their cheek.
When biological samples are collected by the police from a crime scene or an individual, they are sent to a laboratory for analysis. The laboratory extracts the DNA, amplifies it using a chemical reaction, and creates a string of numbers based on part of the sequence of chemical letters: this is known as a DNA profile. The DNA profile is not based on the whole sequence of the DNA (which would currently be very expensive) but on parts of it known as ‘short tandem repeats’ (STRs), where the chemical letters of the
DNA are known to be repeated a different number of times in different people. The final DNA profile consists of a string of numbers based on the number of repeats at each of the STRs, plus the results of a test of the sex of the person from whom the sample came.
DNA profiles are not unique but the probability that two people’s DNA profiles match by chance is low. If the DNA profile from an individual matches the DNA profile from a crime scene it is therefore highly likely (but not certain) that the blood, semen or saliva left at the crime scene came from them.
If the police have a number of suspects for a crime a DNA match can help them to identify who was at the crime scene and who wasn’t. The value of this evidence in solving the crime will vary: DNA on a cigarette butt could have been dropped earlier in the day or have been planted by someone who wanted to implicate an innocent person in the crime; in contrast, DNA in semen from a woman who has been raped can show that a particular man was or was not likely to have been involved. However, even a rape case may not be straightforward: for example, if the man argues that the woman agreed to have sex.
When DNA samples are collected at a murder scene, many DNA matches will occur with DNA from the victim or with others who may have been there earlier in the day, not with the perpetrator of the crime. However, these matches can still help to provide important clues that will help to solve the crime. For example, a DNA match between the victim’s blood and a blood stain on someone’s shoes or clothes might be part of the evidence that leads to a criminal’s conviction. In this example, a DNA database is not required because the victim’s DNA can be obtained easily from him or her.