Lecture 7 - DNA Analysis (continued) Flashcards
STRs
The primary basis of DNA profiles (nuclear DNA) are STRs.
STRs continued
STRs are detected/analyzed through size separation across
the electric field. The more repeats, the heavier the molecular weight, the slower it goes through gel electrophoresis.
Signature in STRs
Number of repeating motifs at a particular loci
CODIS
Combined DNA Index System
primary national DNA database system run by the FBI.
Operates on a local, state, and national level
Used for linking serial crimes and unsolved cases with repeat
offenders
Core set of STR markers - have to be standardized in order to get into the FBI database
Creates a data bank that can be used to search
DNA profiles obtained from crime scene evidence
not for every circumstance:
Cold case hits
* Investigative leads/familial searches
* Missing persons
* STR and mtDNAprofiles from unidentified human remains
* STR and mtDNAprofiles from relatives of missing persons’
Can show familial matches
Small Nucleotide Polymorphisms (SNPs)
changes in the primary
DNA sequence
Substitutions (transitions & transversions), insertions, deletions
where the sequence is different = SNP marker
1 SNP marker isn’t very informative. Need to look at millions of SNPs.
SNPs are the primary basis for mitochondrial DNA.
- BUT—remember that just having a mtDNA sequence match is not
enough - Need to know the population frequency of an entire sequence to
determine how individualizing it is
Phenotype
There are SNPs in nuclear DNA as well, and these can be used to predict an individual’s phenotype. There are known SNPs for hair color, eye color, etc.
These can help narrow down a suspect pool, etc.
If we have a DNA sample, then if we survey all of these SNPs, evidence-staining, extracted DNA, survey tens of thousands, maybe millions of SNPs, can we reconstruct what the person looks like from a blood stain?
The snapshot example. Possibly but it’s not well done.
A better way to test would be a line-up or a face of books.
They all look very white due to using references from the military (DARPA). Now, using Forensic artists to add features from different races/ethnicities.
Why use DNA profiling
At its best, it is the most individualizing biological signature we
have (STRs specifically!)
- Mechanisms of inheritance worked out. Statistics well accepted! We have been studying and measuring the population frequencies.
- Technology to extract, amplify, sequence rapidly gets better and cheaper every year.
What can go wrong with DNA?
Mixtures: Usually have multiple DNA profiles in a sample.
PCR still makes mistakes. Sometimes, the enzyme can slip and add an extra repeat. (Shown as stutter peak in electropherogram).
Degraded DNA: ‘Partial profile’ - especially for TOUCH DNA. This is not very individualizing. Touch DNA is usually really degraded or present in low concentrations.
Peak Heights in electropherogram
Show major/minor contributors.
Issues
There is a lot of opportunities for human error and human bias in DNA profiling. Assumptions are being made when there honestly isn’t enough concrete evidence.
