Biological traces that are submitted for DNA analysis
Depending on whether a biological trace originates from unknown or known persons, it is referred to as undisputable or disputable.
Undisputable biological traces come from a person whose identity is known. This means that the DNA profile is the identification characteristic of that person.
Disputable traces are all other biological traces that are taken from the crime scene. Their analysis establishes forensic DNA profiles.
DNA forensics is a comparative method, i.e. it is based on the comparison of DNA profiles. When a forensic DNA profile is obtained it is then compared with DNA identification profiles of known persons.
What do forensic DNA profiles from biological evidence look like?
Biological traces from crime scenes can originate from one or more persons, so the obtained forensic DNA profile can be unique or mixed. Whether it is unique or mixed is determined by the number of allelic variants per locus.
The presence of more than two allelic variants on a single locus suggests that biological traces were left by several people (mixed DNA profile). The complete DNA profile involves typing all 15 loci. Partial DNA profiles (fewer than 15 loci) are obtained as a result of insufficient quantity and/or degradation of the DNA molecule.
If the disputable biological traces come from more than one person we may get a forensic DNA profile that looks like the one in the table below:
The job of the DNA forensic expert is to obtain the maximum relevant information, and interpret it correctly. Interpretation involves comparison of DNA profiles determined from the disputed trace with DNA profiles of known persons, and a mathematical (biostatistical) analysis.
