dna and trace evidence

DNA

• building blocks of life, present in every cell nucleus. no two dna samples are the same 

• In 1984, sir alec jeffreys discovered first DNA “fingerprint” 

• distinguished every individual in the world

Structure of Nuclear DNA

• in the form of a double helix

• as a result adenine pairs with thymine and guanine pairs with cytosine

  • A-T

  • G-C

Restriction fragment length polymorphism

• RFLPs are length differences with relatively long repeating DNA strands and form the basis for one of the first typing procedures 

Polymerase Chain Reaction Testing 

• PCR offers a distinct advantage in that it can amplify minute quantities of DNA many millions of times 

Short tandem repeats

• most successful and widely used DNA profiling procedure 

• STRs are much less susceptible to degradation and may often be recovered from bodies or stains that have been subjected to extreme decomposition 

• combines both RFLP and PCR

• works on small stains but creates high statistics 

Mitochondrial DNA Testing 

• all individuals of the same maternal lineage will be indistinguishable by mtDNA analysis 

Combined DNA index system

• CODIS is the FBIs software that is used for matching DNA profiles in DNA databases

• the success of the CODIS program is measured by the crimes it helps to solve 

National DNA index system 

• NDIS is the collective database of DNA profiles of the US 

Touch DNA

• most prolific types of human cells 

• leaves epithelial cells 

Dna Probative Value

• Best evidence- DNA belonging to victim or suspect is found where it does NOT belong 

Blood

• ensure there are sufficient samples

• if not, recover for transfer to lab 

• swab the area that appears “clear” these are the white blood cells. There is no DNA in a red blood cell, because there is no nucleus 

Seminal Fluid 

• Alternative Light Source (ALS) can locate possible seminal fluid, followed by presumptive test if you have sufficient samples 

Evidence Collection 

• collect entire item if possible, particularly clothing

• change gloves before handling each piece of new evidence

Minimize contamination 

• Disposable forceps are to be used so that they can be discarded after a single evidence collection. If not disposable, clean with bleach.

packaging biological evidence

• The packaging of biological evidence in plastic or airtight containers must be avoided

DNA evidence collection kits 

• types of evidence collection kits 

  • SANE kit - Sexual Assault Nurse Exam

  • suspect rape lit

  • buccal swab collection kit 

Trace Evidence 

Locard's theory of exchange 

• The intensity, duration, and nature of materials in contact, determine the extent of transfer

• cross exchange bears as a silent witness

Trace Elements 

• May establish the source of a material or at least provide additional points for comparison

Hairs and Fibers

• partial success for individualizing human hairs has been achieved by isolating in characterizing the DNA present in hair 

Comparing Strands of Hair

• when comparing strands of human hair, the criminalist is particularly interested in matching the color, length, and diameter

• The most common request is to determine whether or not hair recovered at the crime scene compares to hair removed from the suspect

Collection and Preservation of Hair 

• 25 full length hairs from all areas of the scalp will normally ensure a representative sampling

• 25 full length pubic hairs should cover the range of characteristics present in pubic hair

• The investigators task of looking for minute strands of fibers often becomes one of identifying and preserving potential “carriers” of fiber evidence

• relevant articles of clothing should be packaged carefully in separate paper bags

• It is necessary to remove a fiber from an object, the investigator must use clean forceps, place it in a small sheet of paper, fold and label the paper, and place the paper packet inside another container

mechanical fit

• mechanical fit is an individualization by fitting two or more pieces of something together: i.e. hit and run car crash

Paint

• One of the most common types of paint examined in the crime laboratory involves finishes emanating from automobiles

• these coatings may include electro coat, primer, primer, surfacer, base coat, and clear coat

Soil

• The value of soil as evidence rest with this prevalence at crime scenes and it’s transferability between the scene and the criminal

• greater individualization may be possible with soil due to contaminants or unique, environmental characteristics

• collect samples of soil from multiple locations believed involved. minimum of six from general area

Two forms of footwear evidence 

• impressions 

• prints- may not be able to physically recover 

Impression Evidence- Tire 

• remove the vehicle from the scene and transport to a secure facility for processing

Tool Mark Evidence

• each tool is manufactured similarly, resulting in class characteristics, such as size shape nature of tool

• but the process of machining or the subsequent use of the tool may leave accidental marks 

• it is the presence of any of my new imperfections on tool that in parts individuality to that tool

• these marks can be as individual as fingerprints

• When practical the entire object or the part of the object bearing the tool, mark should be submitted to the crime laboratory for examination.

• otherwise, cast the tool marks

Impression Evidence: Bite mark 

• two main types of bite mark impressions 

  • ante mortem- less distinction. photograph for several days 

  • post mortem- well defined, no bruising 

• four findings can be related to the examination of bite marks

  • definite – to the exclusion of all others (i.e. only the subject could have made the bite mark impression.)

  • Consistent – no features present will exclude the suspect

  • Possible– due to the nature of the injury, unable to positively confirm or exclude the suspect

  • Exclusionary – definitely not made by the suspect