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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 

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Colin Pitchfork (born 23 March 1960, Newbold Verdon, Leicestershire, England)[1] is a convicted British murderer and rapist. He is the first person convicted of a crime based on DNA fingerprinting evidence, and the first to be caught as a result of mass DNA screening. Pitchfork raped and murdered two girls, the first in Narborough, Leicestershire, in November 1983, and the second in Enderby, also in Leicestershire, in July 1986. He was arrested on 19 September 1987 and sentenced to life imprisonment on 22 January 1988, after admitting both murders.

https://en.wikipedia.org/wiki/Colin_Pitchfork

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Paul Andersen describes the process of DNA fingerprinting and DNA profiling.  He explains how variability in STRs can be used to identify individuals.  He explains the importance of DNA fingerprinting in forensics and paternity cases.

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DNA Definitions

 DNA definitions

Locus is the position of a gene on a chromosome

An allele is one of two or more forms of a gene found at the same place on a chromosome

Genotype is the genetic make-up of a person

Phenotype is the set of observable characteristics resulting from a given genotype

Expression is the appearance of a characteristic attributable to a particular gene

A homozygote has identical alleles for a specified gene

A heterozygote has different alleles for a specified gene on each chromosome

The human genome consists of the total DNA structure, consisting of over 3 billion base pairs

Twenty-two of the 23 pairs of chromosomes look the same and carry the same genes at the same loci. These are called Homologous chromosomes. The exception is the pair of sex chromosomes where the male gender-determining Y chromosome is a different size and shape to the X chromosome.

The key points are:

DNA is located in the chromosomes present in the nucleus of the cell.

The DNA of an individual is the same in every one of his or her cells (but is not present in red blood cells because these cells have no nuclei)and different from everyone else’s other than identical twins.

The DNA molecule resembles a twisted ladder or double helix.

The steps within the ladder are formed by bonding between pairs of 4 bases: guanine (G), adenine (A), thymine (T) and cytosine (C).

(A) always pairs with (T) and (G) always pairs with (C).

Download PDF

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The relationship between nucleus, chromosome, dna, genes, and alleles

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What is Meant by Genetic Difference: a simple overview of DNA inheritance.

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Chemical structure of DNA - a brief overview

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How the area of convergence is calculated blood pattern analysis experts.

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Blood Spatter 101: Blood pattern analysis, examining the basic types of spatter, measuring trajectories, speed, finding the convergence points, etc.

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A visual tutorial on blood pattern analysis.

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Playlist (7 talks): True crime

http://www.ted.com/playlists/269/true_crime

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FIngerprint patterns and ACE-V

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A single incriminating fingerprint can land someone in jail. But, Laura Spinney finds, there is little empirical basis for such decisions.

http://www.nature.com/news/2010/100317/full/464344a.html

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Playlist (5 talks): Talks for understanding the drug trade: http://www.ted.com/playlists/262/talks_for_understanding_the_dr

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Originally published in the American Magazine (1928)

The reader must have equal opportunity with the detective for solving the mystery. All clues must be plainly stated and described.

No willful tricks or deceptions may be placed on the reader other than those played legitimately by the...

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CRIME WRITING - RESOURCES MASTERPOST

Here’s a grand masterlist of crime-related resources. This list is organized into categories, so it is recommended that you take advantage of the CTRL+F function on your keyboard. Let me know if something is amiss, if you have a crime-related post and want it added to this list, or if you want a category added.

WARNING: Links under the cut are not labeled with trigger warnings for images, titles, or mentions of triggering subjects. Please be careful.

This will be updated every time I hoard more links. Last Update: 4/02/15.

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