Biology Reference
In-Depth Information
9
Evaluation and presentation
of DNA evidence
The final stage in any criminal case is the presentation of the evidence to the court.
The way in which DNA evidence is presented has been, and still can be, a con-
tentious subject. The evaluation process and the wording of the court reports and
statements will be affected by both the judicial system and the prevailing approach to
DNA evidence; this varies considerably between different countries. This chapter is
designed as an introduction to the field and will guide the reader through the basics
of the evaluation of DNA evidence.
Hierarchies of propositions
Any statement on the strength of the DNA evidence must be considered in the context
of the case. DNA evidence should not be considered in isolation as it is affected by
many factors such as the type of biological material, method and time of deposition
and the substrate on which it was deposited.
There are three hierarchies of propositions in relation to biological material that
can be considered in a criminal trial [1, 2]:
(1)
Source level
: from which individual did the biological material originate?
(2)
Activity level
: what activity led to the deposition of the biological material?
(3)
Offence level
: did the suspect commit the offence?
The hierarchies can be applied when, for example, considering a bloodstain found
on the clothing of a suspect alleged to have committed an assault. The DNA evidence
may assist in determining the most likely source of the stain. The size, shape and
position of the stain may assist with determining the activity associated with the
stain, that is how was it transferred and does this support an allegation of kicking or
punching. The third hierarchical statement is that of offence: did the suspect commit
the offence? The first hierarchical level can be addressed by DNA analysis and in
many cases the second hierarchical level can be addressed to some extent by the
