Forensic Medicine

Can identify individuals based on phenotype or genotype

Phenotypic identification

Fingerprints

Retinal scan of blood vessel patterns

Blood typing

ABO groups

Based on presence of A or B antigens. Not enough variation to be really useful for paternity testing

HLA analysis

Human leukocyte antigens. Much more variable than ABO

Combination of blood groups and HLA analysis can bring exclusion rate up to 97%. If you are innocent, these tests will prove that in 97% of the cases. If these tests alone are used, there's a chance that you may be convicted even if you're innocent because there's a 3% probability that your analysis matches the evidence even if you're not the perpetrator. Depends on other supporting evidence for conviction.

Genotypic identification

Can often be the only evidence needed for a conviction

Can be obtained from any sample with intact nuclear DNA

2 types of DNA testing:

DNA fingerprinting

PCR amplification

DNA Fingerprinting

Based on RFLPs (restriction fragment length polymorphisms)

1) Digest DNA with restriction enzymes. Have specific target sequences, usually 6 bp. Estimated that 1 nucleotide in 1000 is different between unrelated individuals.

2) Separate fragments according to size (length, MW) by gel electrophoresis with smaller fragments migrating faster through the gel.

3) Perform a Southern blot. Transfer cut DNA to nylon membrane and hybridize with a labeled probe.

4) Make an autoradiograph of the blot

VNTR (variable number of tandem repeats) analysis. Cut out VNTR region, do a Southern and hybridize to VNTR probe. Use several different VNTR regions. DNA fingerprinting is very sensitive to DNA degradation so not as reliable as PCR

PCR Amplification

STR (short tandem repeats) analysis

Loci consist of short, repetitive sequences of 3-7 bp. Allelic forms are differentiated by the number of copies of the repeat sequence contained in the amplification region

Amplification of 3 or more polymorphic STR loci with non-overlapping allele size ranges

Gel electrophoresis to separate products. May either mix amplification products of several different loci or run multiple loci on the same gel. Use of fluorescent-labeled primers allows visualization using a fluorescent imaging device. Products are <400 bp so reliable with degraded DNA samples

With both types of DNA analysis, may have coincidental-match probabilities as low as 1 in 50 billion (i.e. very high power of exclusion)

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SIUC / College of Science / Microbiology / courses / MICR302

URL: http://www.science.siu.edu/microbiology/micro302/forensic.html
Last updated: 9-April-98 / laa