Forensic Biology and DNA

• Author: Cecilia Hageman
• Pages: 381-435

381

A. OVERVIEW OF THE DISCIPLINE: FORENSIC BIOLOGY AND DNA

Forensic biology incorporates the related areas of body f‌luid identif‌ication

(also called serology) and deoxyribonucleic acid (DNA) analysis. Serological

techniques are used to screen evidence items for the presence and type of

body f‌luid deposition. At its most fundamental level, forensic DNA analysis,

like many other forensic tests, is simply a comparison technique that ad-

dresses the question of attribution—can two things, be they two f‌ingerprints,

hairs, f‌ibres, glass, paint fragments, or in this chapter’s case, DNA-containing

body tissue cells, be excluded as originating from the same source? If not, the

two exhibits either came from the same source or from dif‌ferent sources that,

by coincidence, happen to share the same physical or, for DNA evidence, bio-

logical attributes. Compared to other forms of forensic evidence, DNA anal-

ysis of‌fers the clearest statistical picture of the alternative hypothesis—the

probability of a coincidental DNA prof‌ile match is accurately determined us-

ing well-established principles of probability, statistics, and genetics.

Serological analyses target the biochemicals and cells that def‌ine the

distinctive functional characteristics of dif‌ferent body f‌luids, such as he-

moglobin in blood and spermatozoa in semen. The targets of interest in

forensic DNA analysis are microscopic strands of deoxyribonucleic acid,

located in almost every cell in a biological organism. Within long, linear

arrays called chromosomes, DNA contains the molecular instructions to

build and operate a biological organism, be it a microbe, plant, or animal.

CHAPTER 12

Forensic Biology and DNA

Cecilia Hageman

LEgAL ConTExT: CAiTLin PAkosH AnD sARAH HARLAnD-LogAn

382 6 Cecilia Hageman

All humans share mostly the same DNA information, but display minor

variations at some locations, or “loci,” along the chromosomes. Forensic

DNA analysis takes advantage of, and tests for, some of these minor DNA

dif‌ferences to compare and distinguish biological evidence samples.

In 1985, Alec Jef‌freys and his colleagues at the University of Leicester

and the United Kingdom Home Of‌f‌ice Forensic Science Service funda-

mentally changed forensic biology with the introduction of the f‌irst foren-

sic DNA technique, restriction fragment length polymorphisms (RFLPs),

into the analysis of criminal case evidence.1 While forensic biology tech-

niques have progressed far beyond RFLP over the last three decades, they

all work in the same general fashion: isolating the DNA from samples of

biological evidence, targeting known areas of DNA exhibiting minor dif-

ferences, and visualizing these dif‌ferences in the form of DNA prof‌iles.

The evolution in forensic DNA technology has come from the many tech-

nical improvements in targeting and visualizing DNA dif‌ferences, as well

as from signif‌icant gains in standardization, sensitivity, discrimination

potential, and ef‌f‌iciency. Today, for most criminal and civil casework ap-

plications, the majority of forensic DNA laboratories worldwide of‌fer a

technology known as short tandem repeat (STR) analysis. This chapter

will focus primarily on describing and explaining the STR approach to the

forensic analysis of biological evidence samples.

Ef‌fectively prosecuting or defending a forensic DNA case, or review-

ing an old case with a view to DNA retesting, requires at least a rudimen-

tary, and sometimes greater, understanding of the processes underlying

the production and interpretation of prof‌iles, and the role that quality as-

surance plays in assessing and ensuring the reliability of results. Of equal

importance is an appreciation of the statistics surrounding “match” re-

sults, as well as an understanding of the probative value of the presence

or absence of DNA evidence within the context of particular case circum-

stances. This chapter will delve into all of these subjects.

B. THE SCIENCE OF FORENSIC BIOLOGY AND DNA

1) Introduction: From Crime Scene to DNA Prof‌ile

In the laboratory, a forensic DNA analysis starts with a sample of biologi-

cal evidence—a blood stain on clothing, a swab of a beer bottle to gather

1 Peter Gill, Alec J. Jef‌freys, & David J. Werrett, “Forensic Application of DNA ‘Finger-

prints’” (1985) 318 Nature 577 [Gill].

Forensic Biology and DNA 6 383

lip and cheek cells of the drinker, or a cutout of the inside of a baseball cap

to sample forehead skin cells of the wearer, to name just a few examples.

Typical cell types encountered in forensic criminal casework include blood,

semen, saliva (containing the buccal cells from the inside cheeks), nasal

secretions, tooth and bone marrow, vaginal cells, skin cells, and the roots

of hairs.2 The laboratory analysis ends with a prof‌ile picture—a graphical

representation of the DNA variants derived from these cells. This prof‌ile

is interpreted and compared to other similarly produced prof‌iles.

The f‌inal forensic DNA prof‌ile is most simply viewed as a set of num-

bers representing the DNA variants at target locations. These DNA loca-

tions are known as “loci” and are the genetic equivalents of addresses or

postal codes. Genetic dif‌ferences at loci are variants, or “alleles.” A per-

son’s Short Tandem Repeat (STR)-based DNA prof‌ile will look something

like the following example in Table 12.1, where there is a pair of numbers

associated with each genetic address. Each number within a pair rep-

resents a DNA variant (allele) inherited from one of the donor’s two par-

ents. The Xs represent two X sex chromosomes, denoting a female source.3

 12.1. Short tandem repeat prof‌ile produced from a PowerPlex®164 analysis of

a blood sample of a female donor.5 PowerPlex®16 analyzes sixteen genetic address-

es on the human genome, f‌ifteen from chromosome pairs 1 through 22, and one

address from the sex chromosomes. Each number in the pair of results is an allele,

a DNA variant.6

Genetic

address

DNA

STR

alleles

Genetic

address

DNA

STR

alleles

Genetic

address

DNA

STR

alleles

Genetic

address

DNA

STR

alleles

DS   Penta E   DS   v WA  

TH   DS   CSFPO   DS  

DS   DS   Penta D   TPOX  

DS   DS   Amelogenin X X FGA  

2 Urine and feces may contain DNA from cells that line the bladder and intestines, re-

spectively, but are considered to be relatively poor sources for forensic DNA typing

purposes.

3 A male donor would display an X and a Y in his DNA prof‌ile.

4 PowerPlex®16, a product of Promega Corporation (Madison, WI), is one of a num-

ber of commercial kits available to analyze DNA variants for forensic purposes. Iden-

tif‌iler®Plus from Life Technologies (Foster City, CA) also tests f‌ifteen STR locations

plus amelogenin.

5 See Figure 12.1 for the laboratory results from which this interpretation was derived.

6 This is the prof‌ile of the author of this chapter, Cecilia Hageman.