A double-stranded helix (3-D model: requires chime)
Sugar-phosphate backbone outside
Nitrogenous bases (A,C,G, T) inside
Bases held together by hydrogen bonds
A pairs with T: two H-bonds
G pairs with C: three H-bonds
1. The outward appearance of an
organism (phenotype) is influenced
by its hereditary makeup (genotype).
2. Many individual characters
are determined more or less directly by hereditary elements called genes.
3. Genes are located on chromosomes,
each at a particular physical location called a locus.
[BTW, Genes are made of DNA].
4. Alternative forms of genes are
called alleles;
every individual possesses
two
alleles for each gene, one inherited from each parent.
[Individuals with two
identical
alleles are homozygotes;
individuals with two dissimilar alleles are heterozygotes].
5. Some alleles (called dominant)
mask the phenotypic expression of other alleles (called recessive).
Dominant alleles
are symbolized with capital letters (A)
&
recessive
alleles with lower-case letters (a).
(See OMIM
Online Mendelian Inheritance in Man database)
Morphological:
earlobe
attachment
Behavioral:
tongue-rolling
Biochemical
PTC
(phenythiocarbamide) taste sensitivity (demonstration)
The character "PTC sensitivity" is influenced by a
gene (PTC) with
two alleles: "taster"
(T) is dominant to "non-taster"
(t).
~70% of North American whites are "tasters" (TT or
Tt)
(PTC pedigree)
ABO
blood groups
Isoagglutinin locus (I) on Chromosome 9 has
three alleles: IA , IB , IO
A & B
alleles are co-dominant: both are expressed
in the blood group phenotype
A & B
alleles are dominant to O : AA,
AO
& BB,
BO
indistinguishable
Matings ("marriages") between males
& females produce
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six genotypes (AA, AO, BB, BO, AB, & OO) & four phenotypes (A, B, AB, O)
How common
are these alleles, genotypes, & phenotypes in
the human population?
[ Digression into of quantifying
genetic variation ]
In the ABO example:
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1. Forensic blood
/ semen stains
Stain is AB, accused is O: absence of "match" excludes
accused
Stain and accused are both AB;
"match" includes accused,
does not offer positive identification ("proof")
Allele & genotype frequency considerations:
IB is uncommon in western
European populations (<10%),
common in Asian populations (> 30%).
f(IB) ~ 0% in some populations: this may
influence an investigation
2. Paternity testing:
Scenario
1: Suppose mother is Type A, baby is Type B:
Three putative fathers: can any be the actual father?
#1 (Type A): Yes or No?
#2 (Type B): Yes or No?
#3 (Type O): Yes or No?
Scenario
2: Suppose mother is Type A, baby is Type O:
Same three putative fathers: can any be the actual father?
Note that principle is different
3. Multiple-locus
testing
Addition of more loci (e.g., MN, Rh blood factors)
refines probability estimates
MN
bloodtype is determined by two co-dominant alleles
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Crime
Scene scenario: Suppose stain & accused are both Type AB / N
Product Rule: joint probability
of two events is the product of their separate probabilites
prob. of AB and
N blood types = f(AB)
x f(NN) = (0.08) x (0.21) = 0.0168
< 2% of US White population has this combination
Is this sufficient for identification, suspicion, and/or
exclusion?
Juries can become confused by "conflicting" genetic tests:
Mother is Type A / M, Baby is Type B / M, putative Father
is Type B / N
"The
ABO
test says he could be the father, the MN test says
he couldn't :
one of them must be wrong, or should I just ignore both?"
For a good forensic test, choose
multiple
genetic loci with lots
of alleles / locus
All genetic variation eventually comes from DNA molecules
What is the nature of DNA variation?
Is it useful for forensics?
Genes are made of Deoxyribonucleic
acid (DNA)
A double-stranded
helix (3-D model: requires
chime)
Sugar-phosphate
backbone
outside
Nitrogenous
bases
(A,C,G,
T) inside
Bases held together
by hydrogen bonds
A
pairs with T: two H-bonds
G
pairs
with C: three H-bonds
The genetic function of DNA is directly related to its structure
The order of DNA
bases conveys information:
the information is a "genetic code" that specifies protein
sequences
Details are discussed in Biology
2250 (Principles of Genetics)
C A T T A G A C T T G A G
DNA is self-complementary: if you know one strand, you know the other
C
A
T
T
A
G A
C
T
T
G
A G
G
T A A
T C T
? ? ? ? ? ? ? ? ? ?
DNA
therefore has the capacity for self-replication:
in vivo (in living organisms)
[click here for an animation of DNA
replication]
or in vitro (in a "test tube", for example in a forensics
lab)
We will concentrate on what can be learned from the DNA molecule itself
Analysis of DNA involves several
aspects of biotechnology:
"The
use of biological processes to produce goods & services"
Forensic
applications are an example of services:
Polymerase Chain
Reaction (PCR) [Nobel Prize 1993]
"DNA xeroxing":
in vitro (cell-free) DNA "cloning":
Four components & one gadget
DNA
template: anything with
DNA
in it
blood, semen, hair, skin, museum specimen, fossil, etc.
Primers: single-stranded DNA that "stick" to either end
of a gene:
DNA polymerase: DNA replicating enzyme
A, C, G, &
T: four building-blocks
for DNA
Thermal cycler: computer-controlled heating & cooling block
PCR process doubles
gene copy number each cycle:
In
principle: primers initiate copying of new DNA from the
old
(template) DNA,
using the polymerase & the ACGT building blocks
2 
4
8 16
32 64 etc.:
10 cycles 
1,000 copies,
20 cycles 1,000,000 copies,
30 cycles (~3 hrs) 109
copies
[click here for an animation of PCR]
Amplified DNA
can be visualized by gel electrophoresis
Analysis of amplified
DNA
has forensic / scientific applications
Example 1: Restriction Map Analysis
Example 2: Gender determination
in birds
PCR makes sufficient quantities of purified genes for direct analysis by ...
DNA
sequencing [Nobel Prize
1980]
automated
DNA sequencing uses laser fluorometry
modified
ACGTs
are attached to fluorescent dyes (A
C
G
T)
color-coded bases are incorporated into DNA during in vitro
replication reaction
scanning laser & photometer "see"
fluorescence colours
computer assembles four colours, "calls
sequence"
[click here for an animation of automated
DNA sequencing ;
"Helix & Primer"
DNA sequencing service does this locally
DNA
sequences may provide evidence in criminal proceedings:
Same
gene has different sequences in different species
Challenging the "My brother slaughtered a steer"
defence
Species identification in deer
, seals, or tuna poaching.
Different
individuals have different sequences in same species (e.g.
humans)
Example: Differentiation of all ABO
genotypes is now possible at DNA level.
Four variants of O, two of B, one of A => 28 genotypes
in Europeans.
(Johnson
& Hopkinson 1992)
Positive identification of indivduals will probably require ...
DNA
Fingerprinting
Determination of individual-specific
gene patterns
a "fingerprint" cf. dermatoglyphics
VNTR
loci (Variable Number of Tandem Repeats) [aka
mini-
&
microsatellites]
short DNA sequence motifs repeated several times in one place
GCGCGCGCGCGC
(6-fold repeat)
versus
GCGCGC
(3-fold repeat)
Copy number mutates rapidly, even between parents & children
VNTR loci
are highly polymorphic (many different alleles)
=> good markers for within-population
studies
Example: maternity
testing in birds
VNTR
Fingerprint examines multiple loci simultaneously
5 ~ 10 VNTR loci usually provide probabilites < 1 / 10 billion
if band pattern at each of 5 loci has p = 0.01,
joint probability = (0.01)5 = 10-10
Animation
of
DNA Fingerprinting
for forensics
GenCDX Chap 18 animation
for criminalistics, paternity testing (human & equine)
Ethical concerns: GATACCA ?
