Forensic Molecular Genetics Lecture 5 Ralph Kirby

Amount of DNA extractable from a sample is one of the main driving forces in the improvement of forensic DNA profiling The second is the level of discrimination The third is the ease of explanation of the results to a judge or a jury

RFLP Easy to explain But needs lost of DNA Not very high discrimination

Polymerase Chain Reaction (PCR) Able to use very small quantities of DNA Amplifies specific defined DNA sequences –Use specific short single stranded DNA primers –Thermostable DNA polymerase (Fidelity?) –Number of cycles –Detection system Contamination can become a problem

DQA system Involves Specific genes and specific alleles available for such genes Need to avoid gel electrophoresis Uses dot blotting (Southern Blotting) Problem with similarity of markers Use of reverse dot blot with internal control

DQα system in its very early form eliminated suspect 1 as the rapist DQα system as it developed supported Quintanella as the rapist However, this still gave relatively poor discrimination, <10% of the population Would you convict and send to jail for 99 years?

DQα system as it became with 13 probes and 5 typed loci

But hard to explain to a judge or jury

Need for an accurate but obvious system with good discrimination PCR amplification of Short Tandem Repeats Separated on an automatic DNA sequencer Labeled with multiply coloured fluorescent dyes Needs control

This is the type of control used

The STR system for chromosomal markers is fine for DNA samples from most types of cases as they are not mixed However, most DNA from rape cases is a mixture Can separate sperm, but PCR still amplifies victims markers Mixtures reduce discrimination and cause confusion However, in a rape case, only Y chromosome that is present in the sample will be from the rapist (Note possible exception of Y translocations and in utero sex change) Thus STRs for Y chromosome only have been developed Quite good discrimination, especially if used in conjuction with normal STRs

Extremely large number of possible STR markers available Many commercial kits available Some problems with STRs and these will be discussed later

But what about identification from skeletons Mitochondrial DNA