Chapter 8: DNA Forensics

§ In 1975, in England, Dr. Alex Jeffreys developed DNA fingerprinting: the first method of preparing DNA profiles and comparing profiles from different people

§ Minisatellites- variations in the length of certain DNA sequences o Repeated sequences of 10-100 base pairs o Located at many different places on all chromosomes o Used in DNA fingerprinting § Short tandem repeats (STRs)- shorter sequences that are 2-9 base pairs long o Clusters are found widely distributed on all human chromosomes § Single nucleotide polymorphisms (SNPs)- differences in DNA sequence that are only one base pair long and can be specific to an individual § Each of these DNA variations can be used to create a DNA profile and identify individuals o Criminal cases, paternity lawsuits, studies of human evolution, identification of bodies

§ Restriction fragment length polymorphism (RFLP) and polymerase chain reaction (PCR)PCR:§ Done using very small amts of DNA to produce profiles § DNA is heated and cooled § After every round of replication, the amount of DNA is doubled § This can be repeated many times so that a single DNA fragment can be amplified to make millions of copies
RFLP:
§ Done using relatively large amts of blood or other biological samples 1. DNA is extracted from samples 2. DNA is cut into fragments using restriction enzymes 3. DNA fragments are loaded into small slots on gel like substance 4. Electrical current is passed through the gel 5. DNA fragments migrate through the gel & separate by size (smaller fragments migrate faster and further than the larger fragments of DNA) 6. Resulting pattern of DNA fragments is visualized and photographed

§ Each STR is composed of two to nine nucleotides § Each allele of an STR contains a unique number of repeated copies of these nucleotides § STR alleles can show how frequently a specific combo of these alleles is found in a pop § The combined frequency shows that the more alleles analyzed, the rarer the combo becomes in the population

§ Hairs, envelopes, toothbrushes, cig butts, dried saliva, blood, bones, teeth § Evidence from crimes including murder, rape, break-ins, hit and run accidents § Profiles can be prepared from very old samples of DNA as well

§ Scientific evidence does not automatically get admitted into a court of law § Frye v. US separates “good science” from “junk science” § The first case to use DNA forensics began in England in 1983 in Narborough Village Murders § Use of DNA evidence in legal system scrutinized thoroughly How are DNA Profiles Used in the Courtroom? § Forensics- use of scientific knowledge in civil and criminal law § Forensic DNA analysis usually performed in state and local police crime labs, private labs, and Federal Bureau of Investigation lab in WAS DC § DNA is often extracted from biological material left at a crime scene § DNA profiles are prepared from evidence and compared with those of the victim + any suspects § Combined DNA Index System (CODIS) panel is used to develop profile of DNA o If a suspect’s profile does not match that of the evidence, he/she is cleared o 30% of DNA profile results clear innocent people What is a DNA Database? § Catalog of DNA profiles and DNA samples

§ Biohistorians use DNA analysis to identify bodies and body parts of famous and infamous people whose graves have been moved several times or discovered § Used to identify remains of military personnel killed in action (Sept 11, Hurricane Katrina) § Paternity identification: matching samples from mother, father, child allows 100% accuracy in indentifying fathers

§ DNA can be used to trace an individual’s ancestry: mitochondrial & Y chromosome testing § Mitochondrial testing: comparison of haplotypes help trace maternal ancestry o Mitochondria are found only in cytoplasm of cell o Egg is the only gamete with cytoplasm (mitochondria passed from mother to child) o Differences in sequence found in mtDNA provide series of haplotypes o Each person has the same haplotype as his/her mother § Y Chromosome testing: haplotypes are used to trace paternal ancestry o Haplotypes can be grouped to form haplogroups o Y chromosome passed from father to son & est male lineages § Geographic Project: one of largest projects to trace ancestry and patterns of human migration out of Africa and across the globe sponsored by National Geographic Society

§ One has the right to refuse to give a sample in a DNA dragnet § Most states have passed legislation requiring convicted criminals to give samples of DNA § Armed services require members to give samples for identification in case of war § If a prisoner’s case was finalized before 1989, he/she may be allowed to have DNA testing o More than 300 convicted prisoners released by innocence projects § If the crime is murder, no statute of limitation exists and PCR can be used to increase the amt of DNA available for testing