Forensic Characterization of Semen

1. 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

2. 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

3. 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

4. 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)

5. 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.

6. 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)

7. 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.

8. 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

9. 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.

10. 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

11. 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

12. 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.

13. 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.

14. 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

15. 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

16. Collection of Rape Evidence Evidence of rape is not limited to semen • Physical injuries • Blood • Hairs • fibers

17. 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

18. If semen is detected, but contains no spermatozoa, how do you prove presence of semen? • Protein called p30-unique to seminal plasma