Forensic Characterization of Semen  The presence of seminal stains is important in crimes involving sexual offenses. Two steps: 1. Stains must be located 2. Stains are tested to determine identity

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  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  Live (motile) sperm generally survive for up to 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 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 Moisten Stain Collect Stain Open Test Strip Purple =semen o Activate stain with water o Rub stain with provided cotton swab o Test stain by rubbing the moistened swab onto test strip o if it turns purple immediately semen is present(< 30 seconds)

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)  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 —  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 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  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 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  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 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 Evidence Evidence of rape is not limited to semen  Physical injuries  Blood  Hairs  fibers

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