Outline notes for DNA content
https://www.futurelearn.com/courses/introduction-to-forensic-science-3/steps/33147
The main topics
Is it blood (semen) – Understand the role and limitations of presumptive tests as screening tools
Whose blood (semen) – DNA testing – How is the testing done
Who else could it have come from – What for the results mean – Match probabilities – Data bases and CODIS
Is it blood?
Screening tests do not prove blood was present
Confirmatory tests do, but do not provide information on whose blood (or even if it is human)
Same situation with screening for semen
DNA testing
This works by identifying the specific nature of small, discrete segments of DNA
There is not a lot of DNA in a blood stain, therefore the first step is to isolate the segment and replicate it around one million-fold to give enough of the segment to identify its composition
The first step is called amplification and uses a technique call PCR (Polymerase Chain Reaction)
The segments identified in the second step are called Short Tandem Repeats (STRs)
PCR
Mimics what happens in the body
Uses thermal cycler to break DNA double helix into single strands
Locates and “flags” the STR region of interest
Then (taq) DNA polymerase builds the complementary DNA strand to replicate the STR
Process repeated through around 28 cycles
STRs explored
There are hundreds of STR segments in an individual’s DNA
Each is made up of small sequences of DNA joined up end-to-end like a line of bricks
Each STR has its own unique composition of the “brick”
Forensic testing has focused on around 16 of them
The variation within each is in the number of times the “brick” is repeated (hence the “repeat” in the name)
The number of repeats is inherited and the specific version is called an allele
Profiles
The amplified STR alleles are separated from the reagent mixture
Each is compared to a reference sample and the identity of the allele assigned
The quality of the separation technique is critical – you need to know that the preferred method is called CE (Capillary Electrophoresis) but don’t need to know the details
The frequency of each allele for each of the STRs used has been measured in various populations
The DNA profile is the list of STRs and their allele
Whose DNA?
The proportion of people who have a specific STR allele varies somewhat by race and ethnicity
Because the type of one STR does not affect the type of any of the other STRs used in the process, the frequency of the total STR profile can be calculated by multiplying the frequencies of the individual alleles
Data bases
The frequency of occurrence of the various alleles is included in DNA database information
Almost every country has its own database –UK was first, US is largest
The database contains the STR profiles of known offenders and crime samples
The crime samples can be compared to each other to link crimes, and can be compared to the offender data to link the crime samples to a possible source
Other DNA issues
The PCR step can give useable results from just a few cells – Preventing contamination is vital, hence the typical image of the suited CSI
The databases can be used for intelligence, for example where partial matches may lead to identification of a relative as the DNA source
Other DNA systems
Y-STRs Found on the male sex chromosome – passed on from father to son
mtDNA Maternal inheritance Technically more complex than STRs Useful in degraded samples such as Disaster Victim Identification, and where there is little or no nuclear material, such as hair
