The research into the transfer of biological traces aims to investigate whether the traces have a relationship with a committed offence and if so what is the relationship?
Except for the origin of the cells, which can be identified through a DNA-profile the determination of the relationship between the cell donor and the offence is of great significance to evidential value. This relationship can be derived from, for example, the place where the trace is discovered. In addition, the evidential value depends on the nature of the act, the action or the process in which cells are left behind. Therefore a DNA-profile that is obtained from, for example, a sampling of strangulation has more evidential value than the same profile that is obtained from a cigarette butt found at a distance from the victim. After all, the strangulation is in direct relationship with the crime meanwhile the relationship to the cigarette butt is not obvious straight away.
In selecting the traces we shall take into account the likelihood of obtaining a DNA-profile from the sample of evidence. The probability of a proper DNA-profile is much higher from a blood sample than from a touch DNA sample on a random “grabbed” object.
The different types of cells can be subdivided into four categories with reference to the concentration of DNA per volume unit:
- Category 1: semen and tissue;
- Category 2: blood;
- Category 3: cellular material from orifices, such as saliva and nasal moisture;
- Category 4: skin cells.
The type of cells under category 1 are the cells that contain the most DNA per volume unit and from which a full DNA-profile is generally obtained.
Category 4, are the type of cells that contain the least usable DNA per volume unit. Skin cells fall into this category. The chance of obtaining a proper DNA-profile is the smallest in this category.
If the number of samples increases the chance of obtaining a profile will rise. Furthermore the chance of a proper DNA-profile will grow when handling force is increased on the sampled object or when the surface of the sampled object has rough/porous parts. The chance to obtain cells of persons who have been in physical contact with a victim depends on the search strategy (where traces are expected to be found) and the experience of the researcher. The methods used for DNA sampling, DNA-isolation and DNA amplification also play an important role.