1-forensic genetics

Problems with forensic DNA

• Must cope with degraded and touch/trace DNA (poor quality)

• Cope with mixtures of DNA (impure samples)

• Needs to be concordant with ethics/accepted methods/validated against international standards

• Must be compatible with existing databases

• Needs to be cheap

Mini satellites

• Repeat units ~30bp

• Repeat numbers 10s-1000s

• ~10^-2 mutations per generation

Method of DNA fingerprinting

1. Cut DNA with restriction enzymes

   • Contain mini-satellites and isolates short recognition sequence

2. Separate on basis of size (gel) after amplification

   • Longer repeats = longer DNA fragments

3. Transfer to a membrane

4. Probe with a radioactive DNA sequence

   • Detects many minisatellites at once

5. Detected on x-ray film via auto-radiography

Random match probability

• The probability that 2 individuals taken at random from the population will share a DNA fingerprint/profile

  • The smaller the RMP the better

Micro-satellites/short tandem repeats

• 3-6bp repeats

• Repeated ~10-30 times

• ~10^-3 repeats per generation

Y-chromosome/mtDNA

• Y-STRs can be useful in sexual assault cases = pick up male DNA on a female

• MtDNA is useful in degraded and old samples

  • Y-STR allele frequencies can’t be multiplied to calculate RMP = not independently inherited

    • RMPs get quite high and male-like relatives share profiles

  • MtDNA = RMPs quite high and female-line relatives share haplotypes

Identifying the dead via mitochondrial DNA

• Passed from mothers to children

• Survives relatively well in old/damaged material

• Used in forensic casework