1. Chapter 7 DNA Fingerprinting By the end of this chapter you will be able to:
explain how crime scene evidence is
collected and processed to obtain DNA
describe how radioactive probes are used in DNA fingerprinting
explain how DNA evidence is compared for matching
explain how to use DNA fingerprinting to identify DNA from a parent, child, other relative, or a non-related individual
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Forensic Science: Fundamentals & Investigations, Chapter 7

2. DNA Basics Humans have 46 chromosomes (23 pairs)
23 from biological mother, 23 from biological father
23rd pair are sex chromosomes (X and/or Y)
XX = female
XY = male
Forensic Science: Fundamentals & Investigations, Chapter 7

3. Can have extra or fewer sex chromosomes and still survive:
Turner’s syndrome = XO (missing one sex chromosome)
1 in 10,000 births in US
Sterile female, short, has a large neck, no menstruation
Klinefelter’s syndrome = XXY (extra X sex chromosome)
1 in 800 births in US
Males – reduced fertility, delayed motor function, speech, and maturation development
 Supermale syndrome = XYY (extra Y chromosome)
1 in 1000 births in US
Males with normal to below normal intelligence, tall stature
Forensic Science: Fundamentals & Investigations, Chapter 7

4. Supermale syndrome, continued:
Males with XYY sex chromosomes are much more likely to show “impulse outbursts” and to be incarcerated (higher frequency in prison populations)
20 times more likely in prison than in whole population
Led lawyers in the 1960s to attempt a “supermale defense” strategy: “My chromosomes made me do it!”
That’s malarkey!; most XYY males have normal intelligence and are fully functioning in society…
Chromosomes not directly causing violence, but some people argue that the lower intelligence makes them more likely to be caught for their crimes
Forensic Science: Fundamentals & Investigations, Chapter 7

5. Introduction and History of Biological Evidence in Forensics
DNA fingerprinting, also known as DNA profiling, is used in criminal or legal cases with a high degree of accuracy.
Biological evidence includes blood, saliva, urine, semen, and hair AND contains DNA, which can be used for DNA fingerprinting.
Forensic Science: Fundamentals & Investigations, Chapter 7

6. The Function and Structure of DNA
DNA molecules make up chromosome structures and are found in the nucleus of cells in the human body.
How would you describe and explain the double helix, twisted-ladder structure of a chromosome?
Can you explain the terms allele, genome, and junk DNA?
Forensic Science: Fundamentals & Investigations, Chapter 7

7. DNA Identification
Junk DNA does not matter, so it can mutate without consequences and can be used to identify individuals.
In a human population, these differences are called polymorphisms.
In 1984 a technique was developed to analyze polymorphisms.
This DNA Fingerprinting appears as a pattern of bands on X-ray film. These patterns can be used for identification of individuals.
Forensic Science: Fundamentals & Investigations, Chapter 7

8. Variable Number of Tandem Repeats (VNTR)
DNA Identification
The number of copies of the same repeated base sequence in DNA varies among individuals.
Variable Number of Tandem Repeats (VNTR)
Within junk DNA, sequences of DNA are repeated multiple times.
Some can be 9-80 bases in length.
Short Tandem Repeats (STR)
Within junk DNA, other sequences of DNA also are repeated multiple times.
These usually are only 2-5 bases in length and are becoming the preferred sequences for analysis.
Forensic Science: Fundamentals & Investigations, Chapter 7

9. DNA Profiling and DNA Population Databases
VNTR and STR data are analyzed for (a) tissue matching and (b) inheritance matching.
Population genetics is the study of variation in genes among groups of individuals.
Calculations can be made based on these groups to determine the probability a random person would have the same alternative form of a gene (an allele) as (a) a suspect in a crime or (b) an alleged father in a paternity case.
Forensic Science: Fundamentals & Investigations, Chapter 7

10. Sources of DNA
A perpetrator may leave biological evidence, such as saliva or blood, at a crime scene.
This individual evidence is capable of identifying a specific person.
But a small amount of biol. evidence might be trace evidence, and it may be consumed during forensic testing.
In 1993, however, the polymerase chain reaction (PCR) technique was invented. It generates multiple copies of DNA from the biological evidence so that a large enough sample is available for analysis.
Forensic Science: Fundamentals & Investigations, Chapter 7

11. Makes millions of copies of DNA from initial sample by treating it with the correct primers to initiate replication.
Forensic Science: Fundamentals & Investigations, Chapter 7

12. Avoiding contamination in the collection and preservation of DNA
Use disposable gloves and collection instruments.
Avoid physical contact, talking, sneezing, and coughing in the evidence area.
Air-dry evidence and put it into new paper bags or envelopes.
If evidence cannot be dried, freeze it.
Keep evidence cool and dry during transportation and storage.
Forensic Science: Fundamentals & Investigations, Chapter 7

13. Preparing DNA Samples for Fingerprinting
DNA is mixed with special enzymes.
The enzymes cut apart the DNA in specific places forming different sized fragments.
The DNA is loaded into the chambers found on an agarose gel.
An electric current is passed through the gel separating the fragments by size.
Forensic Science: Fundamentals & Investigations, Chapter 7

14. Preparing DNA Samples for Fingerprinting
Extraction
Cells are isolated from biological evidence such as blood, saliva, urine, semen, and hair.
The cells then are disrupted to release the DNA from proteins and other cell components.
Once released, the DNA can be extracted from the cell nucleus.
Forensic Science: Fundamentals & Investigations, Chapter 7

15. Preparing DNA Samples for Fingerprinting
Amplification
With some VNTR analysis, polymerase chain reaction (PCR) can be used to amplify the DNA that contains the VNTRs.
In STR profiles, restriction enzymes are unnecessary; PCR allows the amplification of the strands with STR sequences.
Electrophoresis
DNA samples are treated with digestive enzymes called restriction endonucleases to cut them at specific points.
DNA samples are placed in gels through which electronic currents are passed.
DNA fragments line up in bands along the length of each gel.
Forensic Science: Fundamentals & Investigations, Chapter 7

16. Probes
DNA probes are used to identify the unique sequences in a person’s DNA.
Different DNA probes are made up of different synthetic sequences of DNA bases compli-mentary to the DNA strand.
The probe binds to complimentary bases in the strand (see the fragmentary DNA bands above).
In most criminal cases, 6-8 probes are used.
Forensic Science: Fundamentals & Investigations, Chapter 7

17. Analysis of DNA Fingerprints and Applications
Bands and widths are significant in matching samples of DNA.
DNA fingerprinting can (a) match crime scene DNA with a suspect, (b) determine maternity, paternity, or match to another relative, (c) eliminate a suspect, (d) free a falsely imprisoned individual, and (e) identify human remains.
Forensic Science: Fundamentals & Investigations, Chapter 7

18. DNA Fingerprinting & Paternity Testing
Forensic Science: Fundamentals & Investigations, Chapter 7

19. Mitochondrial DNA
Your cells have two kinds of DNA that illustrate your heredity:
Nuclear DNA, found in the nucleus of most cells (arranged in 46 chromosomes)
Mitochondrial DNA, found in the mitochondria of each cell
Forensic Science: Fundamentals & Investigations, Chapter 7

20. Mitochondrial DNA, cont.
Nuclear – from both parents
Mitochondrial – from mom
Circular DNA
Can trace maternal lineage through mtDNA analysis
Forensic Science: Fundamentals & Investigations, Chapter 7

21. Summary
DNA contains the information needed for replication in a sequence of nitrogenous bases.
DNA analysis allows even a small sample of tissue to be identified with a single individual.
DNA contains, in non-coding regions called junk DNA, many repeated sequences that vary in number between individuals.
These differences between individuals can be used to produce a DNA fingerprint for an individual.
Forensic Science: Fundamentals & Investigations, Chapter 7

22. Summary
Polymerase chain reaction (PCR) for DNA amplification has largely eliminated the problem resulting from the tiny samples usually available.
DNA evidence must be collected carefully to avoid contamination with other DNA.
DNA analysis involves extraction, electrophoresis, and visualization.
DNA profiles are kept by police agencies in electronic databases.
Forensic Science: Fundamentals & Investigations, Chapter 7