DNA Profiling & STR Analysis: STR markers, CODIS, allele interpretation, paternity testing, exclusion vs inclusion, probability calculations
DNA Profiling & STR Analysis:
STR Markers, CODIS, Allele
Interpretation, Paternity
Testing, Exclusion vs Inclusion,
and Probability Calculations
© Syed Hammad Rizvi. All rights reserved.
1/45
,DNA Profiling & STR Analysis: STR markers, CODIS, allele interpretation, paternity testing, exclusion vs inclusion, probability calculations
Introduction & Methodology
Forensic genetics has fundamentally shifted the paradigm of criminal justice, kinship
verification, and human identification. Gone are the days of relying solely on ABO blood
typing or inherently subjective morphological comparisons. The modern era of
identification relies heavily on the molecular nuances of the human genome, specifically
targeting regions of DNA that exhibit profound hypervariability among individuals. This text
provides a rigorous, academically dense examination of DNA profiling, isolating the
precise mechanisms of Short Tandem Repeat (STR) analysis, the interpretative framework
of the CODIS database, and the rigorous statistical mathematics that underpin probability
calculations and paternity testing.
The methodology of this volume bridges the gap between molecular biology and forensic
statistics. To truly master DNA analysis, a student must not only understand how DNA is
extracted, amplified, and separated; they must also possess the analytical acumen to
interpret an electropherogram, differentiate true alleles from biological artifacts, and
calculate the precise statistical weight of an inclusion or exclusion.
This core theoretical guide is structured sequentially. We begin at the molecular level,
examining the architecture of STR markers and their systemic organization within the
Combined DNA Index System (CODIS). Subsequently, we confront the complexities of
allele interpretation, providing a framework for identifying homozygous and heterozygous
states amidst the analytical noise of stutter and degradation. The text then transitions into
the practical application of these profiles in paternity testing, establishing the rigorous
genetic rules that define inclusion versus exclusion. Finally, we anchor these biological
observations in mathematics, utilizing population genetics, Hardy-Weinberg equilibrium,
and Bayesian logic to calculate Random Match Probabilities (RMP) and the Probability of
Paternity.
By engaging with the unique case studies and custom data sets provided herein, the
reader will develop a practitioner-level capability to evaluate forensic evidence, calculate
the statistical rarity of a genetic profile, and present these findings with absolute scientific
objectivity.
2/45
,DNA Profiling & STR Analysis: STR markers, CODIS, allele interpretation, paternity testing, exclusion vs inclusion, probability calculations
Chapter 1: The Architecture of STR Markers
and CODIS
At the core of modern forensic identification lies the Short Tandem Repeat (STR). The
human genome comprises approximately 3.2 billion base pairs, and remarkably, 99.9% of
this sequence is identical across all humans. Forensic science operates entirely within the
remaining 0.1%—the polymorphic regions. STRs are microsatellites consisting of repeating
sequences of 2 to 6 base pairs. The fundamental principle of STR analysis is not the
sequence of the DNA itself, but rather the number of times a specific sequence repeats at
a given physical location, or locus, on a chromosome.
Consider the locus designated as TH01, located on chromosome 11. The repeating motif at
this locus is the tetranucleotide sequence AATG. If an individual inherits a chromosome
from their mother with 7 repeats of AATG, and a chromosome from their father with 9
repeats, their genotype at the TH01 locus is designated simply as 7, 9. The power of STRs
lies in their physical brevity. Because the entire targeted region (including the flanking
sequences where PCR primers bind) is typically only 100 to 400 base pairs long, STRs can
be successfully amplified from highly degraded biological evidence—such as skeletal
remains exposed to the elements or microscopic blood spatter subjected to chemical
cleaning.
To harness the discriminative power of STRs, law enforcement agencies require a
standardized framework. In the United States, this framework is the Combined DNA Index
System (CODIS), administered by the Federal Bureau of Investigation. Originally
established in 1998 with 13 core loci, CODIS was expanded in 2017 to require 20 core loci
for profile upload. This expansion dramatically increased the statistical power of the
database, virtually eliminating the possibility of adventitious (random) matches between
unrelated individuals, even within a database containing millions of profiles.
A complete CODIS profile acts as a digital barcode of human identity. It includes the core
autosomal STR loci, alongside the amelogenin (AMEL) locus, which is utilized for sex
determination. The amelogenin gene possesses a six-base-pair deletion on the X
chromosome compared to the Y chromosome. Consequently, amplification of this locus
yields two distinct fragment sizes in males (X, Y) and a single fragment size in females (X,
X).
Multiplex Polymerase Chain Reaction (PCR) enables the simultaneous amplification of all
20 core loci in a single reaction tube. By utilizing fluorescently labeled primers,
laboratories can separate the resulting DNA fragments by size using capillary
3/45
, DNA Profiling & STR Analysis: STR markers, CODIS, allele interpretation, paternity testing, exclusion vs inclusion, probability calculations
electrophoresis, distinguishing different loci that overlap in size by the distinct colors of
their fluorophores. The architecture of these markers, when analyzed in concert, provides
a discriminatory power that frequently exceeds the current global human population,
cementing STR analysis as the gold standard of forensic science.
Chapter 1: Knowledge Check
1. STR stands for Short Tandem , representing repeating
sequences of 2 to 6 base pairs.
2. The locus utilized primarily for sex determination in standard forensic profiles is known
as .
3. Multiplex allows scientists to amplify multiple genetic loci
simultaneously in a single reaction.
4. The FBI expanded the CODIS core requirement in 2017 to include
total loci to reduce adventitious matches.
5. Due to their small size, typically 100 to 400 base pairs, STRs are highly effective even
when analyzing DNA.
4/45
DNA Profiling & STR Analysis:
STR Markers, CODIS, Allele
Interpretation, Paternity
Testing, Exclusion vs Inclusion,
and Probability Calculations
© Syed Hammad Rizvi. All rights reserved.
1/45
,DNA Profiling & STR Analysis: STR markers, CODIS, allele interpretation, paternity testing, exclusion vs inclusion, probability calculations
Introduction & Methodology
Forensic genetics has fundamentally shifted the paradigm of criminal justice, kinship
verification, and human identification. Gone are the days of relying solely on ABO blood
typing or inherently subjective morphological comparisons. The modern era of
identification relies heavily on the molecular nuances of the human genome, specifically
targeting regions of DNA that exhibit profound hypervariability among individuals. This text
provides a rigorous, academically dense examination of DNA profiling, isolating the
precise mechanisms of Short Tandem Repeat (STR) analysis, the interpretative framework
of the CODIS database, and the rigorous statistical mathematics that underpin probability
calculations and paternity testing.
The methodology of this volume bridges the gap between molecular biology and forensic
statistics. To truly master DNA analysis, a student must not only understand how DNA is
extracted, amplified, and separated; they must also possess the analytical acumen to
interpret an electropherogram, differentiate true alleles from biological artifacts, and
calculate the precise statistical weight of an inclusion or exclusion.
This core theoretical guide is structured sequentially. We begin at the molecular level,
examining the architecture of STR markers and their systemic organization within the
Combined DNA Index System (CODIS). Subsequently, we confront the complexities of
allele interpretation, providing a framework for identifying homozygous and heterozygous
states amidst the analytical noise of stutter and degradation. The text then transitions into
the practical application of these profiles in paternity testing, establishing the rigorous
genetic rules that define inclusion versus exclusion. Finally, we anchor these biological
observations in mathematics, utilizing population genetics, Hardy-Weinberg equilibrium,
and Bayesian logic to calculate Random Match Probabilities (RMP) and the Probability of
Paternity.
By engaging with the unique case studies and custom data sets provided herein, the
reader will develop a practitioner-level capability to evaluate forensic evidence, calculate
the statistical rarity of a genetic profile, and present these findings with absolute scientific
objectivity.
2/45
,DNA Profiling & STR Analysis: STR markers, CODIS, allele interpretation, paternity testing, exclusion vs inclusion, probability calculations
Chapter 1: The Architecture of STR Markers
and CODIS
At the core of modern forensic identification lies the Short Tandem Repeat (STR). The
human genome comprises approximately 3.2 billion base pairs, and remarkably, 99.9% of
this sequence is identical across all humans. Forensic science operates entirely within the
remaining 0.1%—the polymorphic regions. STRs are microsatellites consisting of repeating
sequences of 2 to 6 base pairs. The fundamental principle of STR analysis is not the
sequence of the DNA itself, but rather the number of times a specific sequence repeats at
a given physical location, or locus, on a chromosome.
Consider the locus designated as TH01, located on chromosome 11. The repeating motif at
this locus is the tetranucleotide sequence AATG. If an individual inherits a chromosome
from their mother with 7 repeats of AATG, and a chromosome from their father with 9
repeats, their genotype at the TH01 locus is designated simply as 7, 9. The power of STRs
lies in their physical brevity. Because the entire targeted region (including the flanking
sequences where PCR primers bind) is typically only 100 to 400 base pairs long, STRs can
be successfully amplified from highly degraded biological evidence—such as skeletal
remains exposed to the elements or microscopic blood spatter subjected to chemical
cleaning.
To harness the discriminative power of STRs, law enforcement agencies require a
standardized framework. In the United States, this framework is the Combined DNA Index
System (CODIS), administered by the Federal Bureau of Investigation. Originally
established in 1998 with 13 core loci, CODIS was expanded in 2017 to require 20 core loci
for profile upload. This expansion dramatically increased the statistical power of the
database, virtually eliminating the possibility of adventitious (random) matches between
unrelated individuals, even within a database containing millions of profiles.
A complete CODIS profile acts as a digital barcode of human identity. It includes the core
autosomal STR loci, alongside the amelogenin (AMEL) locus, which is utilized for sex
determination. The amelogenin gene possesses a six-base-pair deletion on the X
chromosome compared to the Y chromosome. Consequently, amplification of this locus
yields two distinct fragment sizes in males (X, Y) and a single fragment size in females (X,
X).
Multiplex Polymerase Chain Reaction (PCR) enables the simultaneous amplification of all
20 core loci in a single reaction tube. By utilizing fluorescently labeled primers,
laboratories can separate the resulting DNA fragments by size using capillary
3/45
, DNA Profiling & STR Analysis: STR markers, CODIS, allele interpretation, paternity testing, exclusion vs inclusion, probability calculations
electrophoresis, distinguishing different loci that overlap in size by the distinct colors of
their fluorophores. The architecture of these markers, when analyzed in concert, provides
a discriminatory power that frequently exceeds the current global human population,
cementing STR analysis as the gold standard of forensic science.
Chapter 1: Knowledge Check
1. STR stands for Short Tandem , representing repeating
sequences of 2 to 6 base pairs.
2. The locus utilized primarily for sex determination in standard forensic profiles is known
as .
3. Multiplex allows scientists to amplify multiple genetic loci
simultaneously in a single reaction.
4. The FBI expanded the CODIS core requirement in 2017 to include
total loci to reduce adventitious matches.
5. Due to their small size, typically 100 to 400 base pairs, STRs are highly effective even
when analyzing DNA.
4/45
