Forensic Biology & DNA Analysis in Criminal Investigations

1. Chapter 13 Biological Fluids: Blood, Semen, Saliva, and an Introduction to DNA

2. Objectives • Students should gain an understanding of: • Tests for the presence of blood • Serological blood typing • Tests for the presence of saliva • Tests for the presence of semen • The principles of paternity • DNA, genes, and chromosomes • Mitochondrial DNA

3. Introduction • Prior to the development of DNA typing, forensic serology was the primary technique of crime labs. • Most labs still use basic serological testing procedures. • Some do not have a DNA typing facility owing to either a lack of resources or a lack of enough cases to warrant the investment.

4. Blood (1 of 2) • Plasma: the liquid portion of blood • Accounts for 55% of the total blood volume • Consists of 90% water and 10% dissolved materials

5. Blood (2 of 2) • Cellular components of blood • Account for 45% of the total blood volume • Include three major types • Erythrocytes: red blood cells • Leucocytes: white blood cells • Thrombocytes: platelets

6. Tests for the Presence of Blood (1 of 6) • Police want to answer three questions: • Is this blood? • Is it from a human? • How closely does it match the blood of the victim or the suspect?

7. Tests for the Presence of Blood (2 of 6) • Presumptive tests for blood • Luminol: sprayed directly on bloodstained object; produces a glow when it contacts blood • Color tests

8. Tests for the Presence of Blood (3 of 6) • Serological tests for blood: precipitin serological test • Determines if blood is of human origin • Can be used with antiserum prepared for other animals if it is negative for human blood • Requires only a small blood sample • May produce a positive result even if bloodstains were washed down to a tiny sample remaining • Is highly sensitive even when bloodstains are odd

9. Tests for the Presence of Blood (4 of 6) • Serological blood typing • ABO system: separates human blood into four broad classifications based on the presence or absence of the antigen A or antigen B on the surface of red blood cells • Rh factor: expressed as positive or negative

10. Tests for the Presence of Blood (5 of 6) • Because blood types are inherited from a person’s parents, blood types may become concentrated among certain ethnic groups. • 80% of the population are secretors—they have significant concentrations of antigens in other body fluids.

11. Tests for the Presence of Blood (6 of 6) • Other blood typing systems: based on the presence of proteins in red blood cells • Polymorphic proteins occur in multiple forms. • Different forms can be identified and their statistical occurrence in the population calculated. • The more independent factors that can be identified in a blood sample, the smaller the percentage of the population possessing that combination of blood traits.

12. Forensic Characterization of Saliva (1 of 2) • Characteristics of saliva • Consists of more than 99% water • pH range = 6.8–7.0 • Contains salivary amylase (a digestive enzyme) • Produced in three main pairs of salivary glands: parotid, submaxillary, sublingual • Cleanses mouth and provides lubrication

13. Forensic Characterization of Saliva (2 of 2) • Saliva is always present at the crime scene if there are bit marks on the victim. • It can be used to identify an individual through DNA profiling.

14. Forensic Characterization of Semen (1 of 2) • Characteristics of semen • Consists of more than 90% water • pH range = 7.2–7.4 • A crime scene may include a large number of items stained by semen (e.g., garments, bed clothing, rugs, drapes, solid surfaces).

15. Forensic Characterization of Semen (2 of 2) • Identification of semen • At the crime scene: UV light • Presumptive tests: acid phosphatase test, p30 test • In the laboratory: direct observation of sperm under a microscope

16. Rape Evidence Collection (1 of 4) • Conviction often hinges on the ability to link the perpetrator to the victim and the victim’s injuries. • Gather evidence from both the physical surroundings and the victim • Collect the victim’s clothing if the victim is still at the scene • Ensure that the victim is examined by a physician immediately

17. Rape Evidence Collection (2 of 4) • Physical evidence collected from the victim: Blood sample Head hair Combings from pubic hair Fingernail scrapings Pubic hair reference samples Oral swab Vaginal swab and smear All clothing Rectal swab and smear Urine specimen

18. Rape Evidence Collection (3 of 4) • Saliva residues • Collect saliva from the victim’s skin if the assailant bit, sucked, or licked an area of the victim’s body

19. Rape Evidence Collection (4 of 4) • Physical evidence collected from the suspect: • All clothing • Combings of pubic hair • Head hair and pubic hair standards • Penile swab • Blood sample

20. Principles of Paternity (1 of 4) • Nucleus: largest structure in a human cell; controls heredity • Ribosomes: site of protein synthesis • Mitochrondria: site of energy production

21. Principles of Paternity (2 of 4) • Chromosomes: hereditary material found in the nucleus • Egg cell: contains an X chromosome • Sperm: contains either an X or a Y chromosome; determines the sex of the offspring

22. Principles of Paternity (3 of 4) • Chromosomes are made of nucleic acids. • Deoxyribonucleic acid (DNA) is the primary hereditary material. • DNA consists of a series of coding regions and noncoding regions that are arranged along the chromosomes. • Genes are sections of the DNA molecule. • Pairs of chromosomes are considered homologous because they are the same size and contain the same information.

23. Principles of Paternity (4 of 4) • When fertilization occurs, one chromosome is inherited from the mother and one is inherited from the father. • Sons inherit their Y chromosome from their father, so paternity can often be determined by comparison of the Y chromosomes from father and son.

24. Introduction to DNA (1 of 13) • Functions of nucleic acids • DNA and RNA are responsible for storage and transmission of genetic information. • They determine how genetic information is transferred from one cell to another and how genetic traits are transferred from parents to offspring. • The major function of DNA is control and direction or protein synthesis in body cells.

25. Introduction to DNA (2 of 13) • Nucleic acids are composed of long chains of repeating units (nucleotides). • Each unit includes three components: • Sugar • Nitrogen-containing heterocyclic base • Phosphoric acid unit

26. Introduction to DNA (3 of 13) • A nucleic acid can contain any of five bases: • Adenine: double-ring base (a purine) • Guanine: double-ring base (a purine) • Cytosine: single-ring base (a pyrimidine) • Thymine: single-ring base (a pyrimidine) • Uracil: single-ring base (a pyrimidine)

27. Introduction to DNA (4 of 13) • Structure of nucleic acids • Adenine, guanine, and cytosine: found in both DNA and RNA • Thymine: found in DNA • Uracil: found in RNA

28. Introduction to DNA (5 of 13) • Primary structure of a nucleic acid: sequence of the four bases • Secondary structure: the double helix

29. Introduction to DNA (6 of 13) • The double helix: each DNA molecule has two polynucleotide chains wound around each other like a spiral staircase • The phosphate–sugar backbone represents the handrails • Pairs of bases linked together by hydrogen bonds represent the steps • Hydrogen bonds hold the two chains together under normal physiological conditions

30. Introduction to DNA (7 of 13) • DNA • Carries the information needed for making and maintaining the different parts of an organism • Chromosomes • Consist of DNA in the nuclei of cells coiled around proteins (histone molecules) • Humans have 46 chromosomes, 23 from each parent • Genes • Are segments of DNA molecules that control the production of different proteins in an organism • Vary in terms of the number and sequence of base pairs they contain

31. Introduction to DNA (8 of 13) • Cell replication • Before a cell divides, the double helix strand begins to unwind. • Each unwinding strand serves as a template for the formation of a new complementary strand. • Nucleotides are attracted to the exposed bases and become hydrogen-bonded to them: A to T, T to A, C to G, G to C.

32. Introduction to DNA (9 of 13) • Ribonucleic acid (RNA) • Primary structure is similar to DNA • Ribose–phosphoric acid units form backbone • Each ribose unit is bonded to one of the four bases • Protein synthesis: a series of complex steps involving RNA • Transcription • Translation

33. Introduction to DNA (10 of 13) • Protein synthesis: transcription • A single strand of RNA is synthesized inside the cell nucleus. • A segment of the DNA double helix separates into single strands. • The exposed bases of one strand act as the template for the synthesis of an RNA molecule. • The base sequence (messenger RNA) complements the base sequence on the DNA strand with one exception: RNA transcribes a uracil instead of a thymine to adenine.

34. Introduction to DNA (11 of 13) • Protein synthesis: translation • The code that has been copied to the new protein is interpreted. • mRNA leaves the nucleus and takes its chemical message to the cytoplasm of the cell, where it binds with ribosomes. • Guided by the first codon on the mRNA strand, a transfer RNA molecule with an anticodon that is complementary to this codon transports a specific amino acid to the mRNA codon

35. Introduction to DNA (12 of 13) • Protein synthesis • The actual protein synthesis occurs in the ribosomes, which move along the mRNA one codon at a time as the amino acid chain grows. • The mRNA is read codon by codon and the protein is built up one amino acid at a time in the correct sequence.

36. Introduction to DNA (13 of 13) • The genetic code • Each three-base sequence in mRNA codes for a specific amino acid • 64 three-base codons can be formed from the four bases in mRNA

37. Nuclear DNA and the Law (1 of 2) • 1985: routine analysis of the structure of human genes led to the discovery that portions of the DNA structure are as unique to each individual as fingerprints • 1987: individuals were first convicted of rape based on DNA evidence (in both the United States and the United Kingdom)

38. Nuclear DNA and the Law (2 of 2) • State v. Woodall: the court accepted the results of DNA testing, but ruled that the inconclusive results failed to exculpate Woodall • Spencer v. Commonwealth: admission of DNA evidence led to guilty verdicts resulting in the death penalty for the defendant • People v. Castro: the court required laboratories and personnel to follow appropriate practices and prove the validity of their procedures before DNA evidence would be accepted in court • DNA testing is generally accepted as admissible under Frye or Daubert standards

39. Mitochondrial DNA • Mitochondria • Provide 90% of the body’s energy • Contain DNA that can be used for testing purposes • Mitochondrial DNA (mtDNA) • Is circular and much smaller than nuclear DNA • Is inherited from the mother • State of Tennessee v. Ware (1996): first use of mtDNA in court to match hair samples at the crime scene to the suspect