BIOL 3000 Genetics and Crime

Restriction Fragment Length Polymorphisms (RFLP)

Molecular technique used to identify variations in DNA sequences.

RFLP Process

Cell sample → DNA is extracted → Cleavage of DNA by restriction enzyme → Separation of DNA fragments by electrophoresis → Transfer to a membrane (Southern blatt) → Binding of radioactive DNA probe to specific DNA fragments → Membrane washed free of excess probe → X-Ray film used to detect radioactive pattern → DNA comparison

Restriction enzymes

An enzyme produced chiefly by certain bacteria, having the property of cleaving DNA molecules at or near a specific sequence of bases. Bacterial proteins such as palindromic sequences

Cleave ae specific DNA (Palindromic Sequences)

Found in prokaryotic organisms as a part of the bacterial immune system

Restriction Sites can be protected from digestion by the addition of Methyl groups

RFLP Analysis

Isolate DNA samples from crime scene and suspects → Treat DNA with Restriction Enzymes → Label DNA with radioactive markers → Electrophorese labeled DNA → Expose to X-Ray film

Variable Number Tandem Repeat (VNTR)

Any location in the genome where a short nucleotide sequence is arranged as a tandem repeat, like microsatellite repeats

Similarities of Restriction Enzymes and VNTR

Isolates DNA from a sample

Electrophorese

Number of specific cut sites in DNA yielding different band patterns

Differences of Restriction Enzymes vs. VNTR

Restriction Enzymes: Cut with enzymes, requires a lot of DNA, longer time to perform and expensive

VNTR: Specific primers to access region of interest (PCR), can use PCR to increase the DNA amount, and it is much quicker and cheaper

DNA Fingerprinting

Most often used to exclude a suspect

Victim vs. Suspects

Short Tandem Repeats (STR)

A microsatellite consisting of a unit of two to thirteen nucleotides repeated several to dozen of times in a row on the DNA strand

STR Analysis

Used to compare specific loci on DNA from two or more samples.

Strand Slippage

Misalignment of repeating STRs → Resulting in the addition or deletion of Repeats (microsatellites) or bases

DNA Fingerprinting - STR

Isolate DNA samples from crime scene and suspects → Amplify DNA using PCR → Label DNA with fluorescent markers → Capillary-Electrophoresis → Laser Detection

Differences of VNTR vs. STR

VNTR: Repeats are 10-100 bp in length, DNA sequence 50-5000 bp in length, many different alleles, found often in heterochromatin, cumbersome and time consuming, hard to tell lots of alleles apart, tend to occur at the end of chromosomes, requires a lot of DNA

STR: Repeats are 2-6 bp in length, DNA sequence < 500 bp in length, Occur all over the chromosome especially in euchromatin, Lend very well to PCR, Number of Repeats is highly variable, Small size allows separation from other areas of the chromosome, Quicker results because Smaller

Combined DNA Index System (CODIS)

A nationwide database managed by the FBI that helps state and local crime labs compare DNA profiles electronically.

DNA Profile

Consists of ONE or TWO alleles for each individual profile at the 13 CODIS Core Loci (+7 recently added). Only data from STR, Y-STR or mtDNA technologies are stored in CODIS.

Loci refers to where the microsatellites happen to be

How CODIS works

Collect DNA evidence → Generate DNA profile → Submit PROFILE to CODIS → Search indices for match

Y-Chromosome STR

All paternally linked males have the same Y-STR profile

Very useful when analyzing evidence with high amounts of female DNA and low amount of Male DNA (sexual assaults)

Application in paternity cases

PARABON DNA Phenotyping

The process of predicting an organism’s phenotype using only genetic information collected from genotyping or DNA sequencing

Test 100,000’s of SNPs based off single nucleotide polymorphisms

Single Nucleotide Polymorphisms (SNPs)

Most common type of genetic variation found in humans

Differences at one specific place

One SNP may occur in ~1% of the population

Almost always biallelic

Are a measure of genetic similarity

Forensic DNA Phenotyping

Prediction of appearance traits of unknown sample donors based upon DNA samples.

Current Externally Visible Characteristics (EVCs): eye, hair, and skin color

Future Externally Visible Characteristics (EVCs): body height/stature, hair vs. baldness, age predictions

Exceptions to DNA Phenotyping

Artificial alterations in appearance

Legal issues

Idea is in its infancy