DNA Fingerprinting. Ray McGovern Nabil Hafez John Leonard. November 29, 2004. Overview. DNA Fingerprinting developed by Sir Alec J. Jeffreys, at the University of Leicester, UK. Leicestershire, saw the first exoneration and conviction bases of DNA evidence, in the late 1980s.
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November 29, 2004
What is DNA Fingerprinting?
Courts & Cases
Probabilities associated with DNA fingerprinting
from locus region
(locus – chromosomal
location of genetic
marker or repeat)
Courtesy: Fan Sozzi
Older method (80’s)
Less sensitive – suitable for blood, requires large amount of high quality DNA
Slow: 7-10 days, labor intensive, one gene analyzed at a time
Fewer indvidual tests needed, genes examined have >100 alleles high power of discrimation
Newer method (90’s)
More sensitive – can use degraded DNA from minute sources
Fast: 2-6 days, high throughput, simultaneous analysis of many genes
More individual tests needed, genes examined have between 10-15 alles, low power of discrimination
When determining the admissibility of DNA evidence, courts consider the following questions:
The majority of the time, “the possibility of laboratory error is substantially larger than the possibility of a coincidental match. This is not because DNA laboratory work is particularly sloppy or unreliable. Instead, it is because the chance of a coincidental match is usually small.”
Dr. Schneeberger, a Canadian doctor who hading been acquitted of sexually assaulting in 1994, had giving blood on three occasions. He had surgically inserted a Penrose drain, filled with foreign blood and anticoagulants in his arm.
This only came to light when the police obtain a sample of his hair and Dr. Schneeberger was convicted in 1995.
Houston Police Department’s Crime Lab
Cellmark was awarded a $2.7 million three-year contract to provide DNA testing service to the LA Police Department, in January.
In November, Sarah Blair a DNA analyst was fired for substituting data in control samples causing Cellmark to reanalyzed the evidence of several high-profile cases; O.J. Simpson, JonBenet Ramsey, the Unabomber, etc.
The admissibility of scientific evidence in the federal court was established Frye v. United States, 1923. The Frye court put forward the "general acceptance" test.
State v. Stills – the Supreme Court of New Mexico approved the admissibility of PCR analysis stating that PCR analysis "has received overwhelming acceptance in the scientific community and the courts".
The Product Rule
“The product rule requires an assumption of within (Hardy-Weinberg or HW) and between (linkage equilibrium or LE) locus independence, which cannot be exactly true.”
The National Academy of Sciences first recommended the use of the Ceiling Principle back in the late 1980s. The idea was to provide the most conservative estimate of probability but in the mid 1990 it fell out of favor
Advances in DNA fingerprinting
A Shift in the scientific landscape,
Who was Hardy-Weinberg?
Cells, Chromosomes, Genes
23 pairs of chromosomes
Each set passed from parents, one from each
Inherited characteristics controlled by genes
Genes can come in alternative forms, alleles
Each controls specific trait, example: eye color
brown, green, blue
One allele from each parent
Dominant vs. Recessive
Recessive: Sickle Cell Anemia
Allele causes shape of red blood cells to be altered
2 copies - disease is manifested
1 copy - carrier
Progressive, usually mid 30’s
Involuntary muscular action and dementia
Only 1 copy of allele needed
What happens when at a given locus
Both Dominant & Recessive alleles present
Godfrey Harold Hardy - mathematician
Wilhelm Weinberg - physician
Mathematical equivalent of Mendel’s observations
For a gene with 2 alleles, A and a
p the population frequency of A
q the population frequency of a
AA = 25%Aa = 50%aa = 25%
Population size infinite
No selection for or against genotypes
No net migration into or out of population
match but need to relate to
Each allele has an observed frequency
Use the product rule
Blond hair: 1/10
(.10) (.10) (.10) = 1/1000
What about in Scandinavia? Issue of population
95% confidence limit, true value has only
5% chance exceeding upper bound
If pattern 1/100
Upper confidence limit4.7%
Group I - 90%.5.5
Group II - 10%.9.1
with random mating
Without regard to population substructure
Sickle Cell Anemia
observed frequency : 0.09
p + q = 1
q = % population with SCA = .09
p = % population with out SCA = .91
Hardy-Weinberg Equilibrium is a powerful tool
Pay attention to its constraints