Transcript GLYPHOSATE RESISTANCE Background / Problem

Lecture 15: Forensics: Probability of
Individual Identity
March 5, 2014
Genetic Typing in Forensics
 Highly polymorphic loci provide unique ‘fingerprint’
for each individual
 Tie suspects to blood stains, semen, skin cells, hair
 Revolutionized criminal justice in last 20 years
 Also used in disasters and forensic anthropology
 Principles of population genetics must be applied in
calculating and interpreting probability of identity
Markers in Genetic Typing
 Standard set of 13 core loci for
forensics: CODIS (Combined DNA
Index System)
 Sets of highly polymorphic
microsatellites (also called VNTR
(Variable Number of Tandem
Repeats), STR (Short Tandem
Repeat) or SSR (Simple Sequence
Repeat))
http://www.cstl.nist.gov/div831/strbase//mlt_abiid.htm
 Most are amplified in a single multiplex reaction and analyzed in a
single capillary
 Very high “exclusion power” (ability to differentiate individuals)
Individual Identity: Likelihood
 Assume you find skin cells and blood under fingernails of
a murder victim
 A hitman for the Sicilian mafia is seen exiting the
apartment
 You gather DNA evidence from the skin cells and from
the suspect
 They have identical genotypes
 What is the likelihood that the evidence came from the
suspect?
P(G | H1 )
L( H1 , H 2 | G )  LR 
,
P(G | H 2 )
 What is H1 and what is H2?
Match Probability
 Probability of observing a genotype at locus k by
chance in population is a function of allele frequencies:
Pk  p
Pk  2 pi p j
Homozygote
Heterozygote
2
i
m
P   Pk
for m loci
k 1
 Assumes unlinked (independent loci) and HardyWeinberg equilibrium
Probability of Identity
 Probability 2 randomly selected individuals have same
profile at locus k:
PIDk   pi4   (2 pi p j ) 2
i
Homozygotes
i
i j
Heterozygotes
m
P   PIDk
for m loci
k 1
 Exclusion Probability (E): E=1-P
Which allele frequency to use?
 Human populations show
some level of
substructuring
Illinois Caucasian
FST generally < 0.03
 Challenge is to choose
proper ethnic group and
account for gene flow
from other groups
Georgia Caucasian
U.S. Black
http://books.nap.edu/openbook/0309053951/gifmid/95.gif
Substructure in human populations
 GST is quite high among the 5 major groups of human populations
for CODIS microsatellites
 Relatively low within groups, but not 0!
NRC (1996) recommendations
 Use population that provides highest probability of
observing the genotype (unless other information is
known)
 Correct homozygous genotypes for substructure within
selected population (e.g., Native Americans, hispanics,
African Americans, caucasians, Asian Americans)
 No correction for heterozygotes
P'  [ pi2  pi (1  pi )FST ] 2 pi p j
Homozygotes
Heterozygotes
Why is it ‘conservative’ (from the standpoint of
proving a match) to ignore substructure for
heterozygotes?
HT  H S
FST 
HT
H S = HT (1- FST )