Genes and identity

• Uses:

  • Forenisc analysis
  • paternity analysis
  • missing preson/disaster victim (may use mitochondrial DNA)
  • Genealogy and deep ancestry
  • population migration patterns
• What is the goal of forensic science?

  To unambiguously (100% guarantee) match biological specimen left at the scene of a crime to specific individuals
• Examples of forensic:

  Blood

  Semen

  Hair
• What does genetic identification require?

  Minisatellites (variable number tandem repeats – VNTRs)

  Microsatellites (short tandem repeats – STRs)

  Single nucleotide polymorphisms (SNPs)

  Can examine autosomes, Y chromosome or mitochondrial DNA (mtDNA)
• Minisatellites

  • discovered by Alec Jeffreys.
  • led to the development of DNA fingerprinting
  • Found in introns
  • Short (7-100 repeats) identical nucleotide sequences arranged tandemly head to tail
  • Each minisatellite located at a locus on a particular chromosome
  • Inherited in Mendelian fashion (one from each parent)
  • Number of repeats at a locus varies between individuals – variants known as alleles
• How to form a DNA fingerprint from VNTRS

  Using the core as a probe detects multiple VNTRs simultaneously (multilocus probe)

  Differences between individuals in repeat number at each locus
• How does paternity test work?

  Child receives half chromosomes and therefore half VNTR alleles from each parent.
• Western blot

  uses polyacrylamide gel

  put on to nylon membrane, fixed

  DNA is labelled (probe)
• Limits of multilocus probes:

  bands produced cannot be assigned to any particual location (locus)

  DNA fingerprint not readily amenable to statistical analysis whihc hampered it use in court
• Disadvantage of southern blot:

  1. Need to have sufficient biological material (typically DNA content of 1000 cells)

  2. Multistep procedure - isolate DNA, digest DNA, run DNA on gel, Southern blot, probe membrane, identify bands hybridising to probe

  3. Good quality DNA required (sensitive to contaminants and degradation)

  The use of the Polymerase Chain Reaction (PCR) overcomes these limitations and has revolutionised DNA testing
• PCR and forensics:

  Need very little starting material

  Starting material doesn’t need to be high quality: works best with short target sequences – shorter VNTRs or Short Tandem Repeats

  Simple, rapid procedure, producing results in a few hours

  Individual loci analysed – allele frequency and statistical data available
• Microsatellites:

  • Microsatellites – also called short tandem repeats (STRs) are a simpler version of VNTRs.
  • Repeated sequence 2-6 nucleotides e.g. CACACACACACA

  STRs present throughout human genome in both coding (exons) and non-coding (introns) regions at known loci

  • STRs typically not as hypervariable as VNTRs, but many loci are known.
  • STRs amplified using  PCR with primers designed to anneal to unique flanking sequence
• How to analsise multiple STRs at once

  use fluorescent dye labelled primers
• STRs

  1. Extract DNA
  2. Amplify DNA containing STRs with up to 10 pairs of primers. Label one of each primer pair with different fluorescent dyes to allow identification of product.
  3. Separate products – Interpret results

  Uk uses 10 STRs
• UK examples of DNA databases

  Ipswich murders: 

  Steve Wright convicted in  Feb 2008 of murders of 5 people.

  Wright caught after DNA found on 3 bodies matched a sample he had given 4 years earlier when arrested for theft.

  Croydon murder:

  Mark Dixie arrested and convicted after DNA taken after his arrest following a pub brawl 9 months after Sally Anne’s murder proved to be identical to that  left on Sally Anne’s body
• Issues with DNA bases:

  UK has largest DNA database in the world

  • Uses standard set of 10 STRs
  • Other European countries use at least 16 with more sensitive PCR

  Until recently, all DNA samples taken during criminal inquiries in England and Wales were retained, but Protection of Freedoms Act 2012 outlaws retention of samples from innocent individuals.
• Y chromosome analysis:

  • used if small amount of male DNA mixed with larger amount of female DNA e.g semen, touch DNA, male skin beneath female nails
  • Y STR inherited together (need to compare Y haplotypes)
  • can be used to determine paternal bio-geographic ancestry and paternal male relative (useful cold cases)
• Surname relationship with Y chromosome:

  Men with the same (less common) surname tend to share Y chromosome markersth century – Most Sykes men share a common ancestor and Y chromosome

  Exceptions arise through non-paternity e.g adoption

  Example: Sykes surname originated near Huddersfield in 13
• mtDNA analysis:

  Useful in old or degraded sample

  Nucleotide sequencing of hypervariaable regions - expensive and slow
• Can SNPS be used to predict face shape?

  Partially, 12 genes are associated.