Forensic Evidence

• Author: Daniel Brown/Jill Witkin
• Pages: 269-291



Forensic

Evidence

10

I. Forensic DNA in Sexual Oence Investigations, Prosecutions,

and Defences ............................................ 

II. Understanding the Technology and DNA Prole Interpretation .... 

A. Documentation ..................................... 

B. Body Fluid Testing ................................... 

C. Forensic DNA Analysis ............................... 

III. Understanding Transfer and Persistence of

DNA-Containing Cells in the Context of a Case ................. 

IV. Paternity Analysis in Sexual Oence Investigations .............. 

This chapter was authored by Cecilia Hageman.

© [2020] Emond Montgomery Publications. All Rights Reserved.

 Prosecuting and Defending Sexual Oence Cases

I. Forensic DNA in Sexual Oence Investigations,

Prosecutions, and Defences

Biological evidence plays a crucial role in the investigation, prosecution, and defence

of many sexual oences allegations. Biological evidence is mobile evidence, and

physical exchanges of blood, skin, semen, and epithelial1 and buccal2 cells may lead

to meaningful inferences regarding sexual (and other) physical contact and activity

between persons or between persons and objects. Cells that contain deoxyribonucleic

acid (DNA) may be recovered from victims or accused persons, or both, from body

cavity, skin, and other intimate samples, bite marks, undergarments, outer clothing,

and fingernails. Biological evidence may be present at the crime scene on condoms,

bedding, furniture, and carpets. The transfer of semen cells via vaginal intercourse

may also be evidenced through a paternity analysis, where the DNA profile of a child

or aborted product of conception arising from an alleged sexual oence is analyzed

to determine the contribution from the semen donor, allowing for a comparison to

potential male suspects.

Forensic scientists tasked with examining sexual oences case evidence first review

case circumstances and available evidence items to identify the questions of interest to

investigators. They then determine the relevant hypotheses in order to decide which

scientific tests to carry out. A hypothesis can be considered an allegation to be tested

using scientific methods. It is therefore important to begin this discussion with general

types of hypotheses that a scientist can address in sexual oences cases. Scientists ali-

ated with the (former) Forensic Science Service in the United Kingdom provided a very

useful “hierarchy of propositions”3 to help define and set apart the dierent categories

of questions that investigators and legal counsel address, and how scientists can define

the proper hypotheses and testing conditions for these questions.

The highest of four proposition levels speaks to the probability that the suspect

committed a criminal oence. This consideration of the ultimate issue, the compet-

ing hypotheses of guilt and innocence, is entirely beyond the capability and realm of

the scientist. Moving downward and in order, the remaining three proposition levels

(listed in the table below) deal with questions of activity (i.e., “What happened?”),

body fluid deposition (i.e., “What and whose body fluid was deposited?”), and DNA

source (i.e., “Who is the source of DNA from some indeterminate cell deposition?”).

In a typical sexual oence case where DNA profiles are obtained from an evidence

item and from a reference suspect, for example, competing hypotheses and typical

forensic DNA evidence may include those outlined in Table 10.1.

1 These are cells that line orifices such as the vagina and the rectum.

2 Buccal cells line the inside of the mouth and can be found in saliva.

3 R Cook et al, “A Model for Case Assessment and Interpretation” (1998) 38 Science &

Justice151; R Cook et al, “A Hierarchy of Propositions: Deciding Which Level to Address in

Casework” (1998) 38 Science & Justice 231.

© [2020] Emond Montgomery Publications. All Rights Reserved.