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Forensic Characterization of Semen. The presence of seminal stains is important in crimes involving sexual offenses. Two steps: Stains must be located Stains are tested to determine identity. Testing for Seminal Stains.

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forensic characterization of semen
Forensic Characterization of Semen
  • The presence of seminal stains is important in crimes involving sexual offenses.

Two steps:

  • Stains must be located
  • Stains are tested to determine identity
testing for seminal stains
Testing for Seminal Stains
  • Seminal stains may be visible on fabric due to their stiff, crusty appearance.
  • Acid phosphatase test is the best way to locate and characterize seminal stains.
  • Once sample is proven to be semen, the next step is to associate the semen as closely as possible with an individual
microscopic examination of semen
Microscopic Examination of Semen
  • Semen is unequivocally identified by the presence of spermatozoa.
  • Usually easy to locate sperm in semen
  • Reasons why sperm might not be found
    • Sperm bind tightly to cloth material
    • Sperm are extremely brittle when dry and easily disintegrate when washed or rubbed against another object
    • Oligospermia — lows sperm count
    • Aspermia— no sperm in seminal fluid
seminal constituents sperm
Seminal constituents — sperm
  • Live (motile) sperm generally survive for up to 4 - 6 hours in the vaginal cavity
    • Vaginal smear must be examined microscopically immediately after it is taken from the victim
  • Nonmotile sperm may be found up to 3 days in the vaginal cavity (occasionally up to 6 days later)
  • Intact sperm (sperm with tail) are not normally found 16 hours after intercourse (but have been found 72 hours later)
acid phosphatase test
Acid Phosphatase Test
  • Acid phosphatase is an enzyme secreted by the prostate gland into seminal fluid.
  • Concentration is 400X more in seminal fluid than in any other body fluid.
  • A reaction time of less than 30 seconds is a strong indication of semen.
testing seminal stains with acid phosphatase
Testing Seminal Stains with acid phosphatase
  • Moisten Stain Collect Stain Open Test Strip Purple =semen
  • Activate stain with water
  • Rub stain with provided cotton swab
  • Test stain by rubbing the moistened swab onto test strip
  • if it turns purple immediately semen is present(< 30 seconds)
testing for seminal stains1
Testing for Seminal Stains
  • Many of the cases sent to a forensic laboratory involve sexual offenses, making it necessary to examine exhibits for the presence of seminal stains.
  • The best way to locate and at the same time characterize a seminal stain is to perform the acid phosphatase (an enzyme secreted into seminal fluid) color test.
    • A purple color indicates acid phosphatase enzyme.
  • Semen can be unequivocally identified by either the presence of spermatozoa or of p30, a protein unique to seminal plasma.
  • Forensic scientists can successfully link seminal material to an individual by DNA typing.
prostate specific antigen psa or p30
Prostate Specific Antigen (PSA or p30)
  • Positive acid phosphatase test but can’t find any sperm
    • how can you prove unequivocally that it’s semen?
  • By use of p30 (prostate specific antigen, PSA)
    • Antigen — antibody reaction

Figure 8-18  An antibody–antigen–antibody sandwich or complex is seen as a colored band. This signifies the presence of PSA in the extract of a stain and positively identifies human semen.

seminal constituents
Seminal constituents —
  • Finding acid phosphatase decreases with time after intercourse
    • Little chance of identifying it after 48 hours
  • Need to know if voluntary sexual activity occurred before the assault
  • p30 is NOT normally found in the vaginal cavity beyond 24 hours after the assault
testing for seminal stains2
Testing for Seminal Stains
  • Microscopic Examination- Semen can be identified by the presence of spermatozoa
  • Prostate Specific Antigen – antigen/antibody reaction confirms the presence of seminal fluid
rape evidence
Rape Evidence
  • The rape victim must undergo a medical examination as soon as possible after the assault.
  • At that time the appropriate items of physical evidence including clothing, hairs, and vaginal and rectal swabs can be collected for subsequent laboratory examination.
  • All outer and undergarments should be carefully removed and packaged separately in paper (not plastic) bags.
  • Bedding, or the object upon which the assault took place, may also be carefully collected.
rape evidence1
Rape Evidence
  • If a suspect is apprehended within 24 hours of the assault, it may be possible to detect the victim’s DNA on the male’s underwear or on a penile swab of the suspect.
  • Items routinely collected from the suspect include all clothing, pubic hair, head hair, penile swab, and a blood sample or buccal swab for DNA typing.
  • The forceful physical contact between victim and assailant may result in a transfer of such physical evidence of blood, semen, saliva, hairs, and fibers.
how to protect rape evidence
How to protect rape evidence
  • Outer garments and undergarments carefully removed and packaged in separate paper bags. WHY?
  • Don’t fold an article through a seminal stain as it may damage the sample.
  • Latex gloves must be worn when collecting samples
collection of rape evidence
Collection of Rape Evidence

Physical evidence collected from a rape victim include:

  • Pubic combings
  • Pubic hair reference samples
  • External genital dry-skin samples
  • Vaginal swabs
  • Cervix swabs
  • Rectal swabs
  • Oral swabs
  • Head hairs
  • Blood sample
  • Fingernail Scrapings
  • Clothing
  • Urine specimen
collection of rape evidence1
Collection of Rape Evidence

Evidence of rape is not limited to semen

  • Physical injuries
  • Blood
  • Hairs
  • fibers
review q
Review Q:
  • What are three reasons why spermatozoa are often not found in seminal fluid collected at a crime scene?
    • bind tightly to cloth
    • extremely brittle when dry and easily disintegrate if the stain is washed or rubbed
    • sexual crimes may involve males who have abnormally low—or even zero—sperm count

If semen is detected, but contains no spermatozoa, how do you prove presence of semen?

  • Protein called p30-unique to seminal plasma